Custard square (often called custard slice or vanilla slice) is a bakery-style dessert made from two crisp sheets of puff pastry with a thick layer of set vanilla custard in the middle, finished with a simple vanilla icing. This version follows the same components + weight-based approach used in the recipe you shared from Cloudy Kitchen (puff pastry, custard filling, icing), but the instructions below are written fresh and original.

Yield + Time

• Yield: about 12 squares
• Best plan: make over 2–3 days (puff pastry → bake + fill → chill → ice + slice).

Ingredients (with grams)

1) Homemade Puff Pastry

Butter block

• 450 g unsalted butter, room temperature
• 70 g bread flour

Dough (détrempe)

• 400 g bread flour
• 200 g all-purpose flour
• 1½ tsp fine sea salt
• 115 g unsalted butter, room temperature
• 1 Tbsp vanilla bean paste (optional)
• 290 g cool water

2) Vanilla Bean Custard Filling

• 720 g whole milk
• 480 g heavy cream
• 2 tsp vanilla bean paste
• pinch of salt
• 65 g custard powder (see note below for cornstarch swap)
• 200 g sugar
• 3 eggs, room temperature
• 70 g unsalted butter, room temperature

3) Vanilla Icing

• 375 g powdered sugar, sifted
• 45 g unsalted butter, room temperature
• ½ tsp vanilla bean paste
• whole milk, a few tablespoons (as needed)

Equipment (helpful)

• Rolling pin + ruler
• 2 sheet pans
• Parchment paper
• Another sheet pan (to “weight” pastry while baking)
• 9×13 in (23×33 cm) pan or similar slice pan
• Whisk + medium saucepan
• Fine sieve (optional, for extra-smooth custard)

Step-by-Step Instructions

Day 1 (optional but recommended): Make the puff pastry

Step 1 — Make the butter block

1. In a bowl, mix 450 g butter with 70 g bread flour until smooth and fully combined.
2. Shape into a neat rectangle (about 1–2 cm thick), wrap well, and refrigerate until firm but pliable (it should bend slightly without cracking).

Step 2 — Make the dough

1. Mix the bread flour, all-purpose flour, and salt in a large bowl.
2. Rub in the 115 g butter until you have a sandy texture with some small butter pieces.
3. Add cool water (and vanilla paste if using). Mix just until a dough forms.
4. Shape into a rectangle, wrap, and chill 30–60 minutes.

Step 3 — Lock in the butter

1. Roll the dough into a rectangle large enough to wrap around the butter block.
2. Place the butter block in the center and fold the dough around it, sealing edges so the butter is fully enclosed.
3. Chill 20–30 minutes if the butter feels soft.

Step 4 — Do your turns (folds)

You’re building layers. Work on a lightly floured surface and keep everything cold.

1. Roll the dough into a long rectangle.
2. Turn 1: do a 4-fold (book fold) → chill.
3. Turn 2: do a 3-fold (letter fold) → chill.
4. Turn 3: 4-fold → chill.
5. Turn 4: 3-fold → chill.

Tip: If the dough resists rolling or the butter starts to melt, stop and chill. Clean layers = flaky pastry.

Wrap tightly and refrigerate overnight (or freeze for later).

Day 2: Bake pastry sheets + cook custard + assemble

Step 5 — Roll and bake two pastry sheets

1. Heat oven to about 200°C / 400°F (a typical puff pastry baking temp).
2. Divide puff pastry into two equal pieces.
3. Roll each into a thin rectangle sized to fit your pan (or your desired final slice size).
4. Place each sheet on parchment-lined baking trays.
5. Dock all over with a fork (helps control puff).
6. Place another sheet pan on top to weight the pastry so it bakes flat and crisp.
7. Bake until deep golden and fully crisp. Cool completely.

If one tray finishes earlier, remove it—overbaked pastry can taste bitter.

Step 6 — Cook the vanilla custard

1. In a saucepan, warm milk + cream + vanilla + salt until steaming (not boiling).
2. In a bowl, whisk custard powder + sugar first (this helps prevent lumps), then whisk in the eggs until smooth.
3. Slowly pour a little hot milk/cream into the egg mixture while whisking (tempering).
4. Pour everything back into the saucepan.
5. Cook over medium heat, whisking constantly, until thick and bubbling (you want it to fully activate the starch so it sets well).
6. Remove from heat and whisk in the 70 g butter until glossy.
7. Optional: pass through a sieve for ultra-smooth custard.
8. Let custard cool 10–15 minutes so it’s thick but still spreadable.

Custard powder note: the Cloudy Kitchen post uses custard powder and mentions cornstarch as a substitute if needed. A common swap is using cornstarch in a similar quantity by weight, though results can taste slightly less “custard-y.”

Step 7 — Assemble the custard square

1. Line your pan with parchment, leaving overhang for lifting.
2. Place one baked pastry sheet in the bottom (trim to fit).
3. Pour/spread the custard evenly over the pastry.
4. Add the second pastry sheet on top, pressing gently to level (don’t crush it).

Step 8 — Chill to set

Refrigerate at least 4 hours, but overnight is best for clean slices and a fully set custard.

Day 3 (or later): Ice + slice

Step 9 — Make the vanilla icing

1. Mix powdered sugar + butter + vanilla.
2. Add milk little by little until you get a thick, spreadable icing (it should slowly settle smooth, not run).

Step 10 — Ice the top

Spread icing over the top pastry layer. Chill 15–30 minutes to set the icing.

Step 11 — Slice neatly

For clean slices:

• Use a sharp serrated knife for the top pastry.
• Wipe the knife between cuts.
• You can score the top pastry first, then cut straight down.

Lift out using parchment overhang, then cut into squares.

Storage

• Keep refrigerated in an airtight container 2–3 days (pastry softens over time, but it’s still delicious).
• Best texture: day 1–2 after assembly.

Shortcut Options (if you want)

• Use store-bought puff pastry to save time (still great).
• Or make rough puff (faster than classic puff) if you’re comfortable with it—Cloudy Kitchen also discusses these alternatives.