Preheat oven to 400°F (200°C). Line a baking sheet with parchment. Cut thawed puff pastry sheet in half to get two equal rectangles.

Place both rectangles on the baking sheet. Prick all over with a fork. Bake for 15-20 minutes until golden brown and crisp. Cool completely on a wire rack.

In a large saucepan, heat 3.5 cups of the milk over medium heat until scalding (steaming but not boiling).

In a large bowl, whisk together sugar, cornstarch, and salt. Whisk in the remaining 0.5 cup of cold milk until a smooth paste forms. Whisk in the 6 egg yolks.

Very slowly, pour about 1 cup of the hot milk into the egg yolk mixture, whisking constantly. Then, pour this tempered egg mixture back into the saucepan with the rest of the hot milk.

Return the saucepan to medium heat. Whisk constantly and vigorously until the mixture bubbles and becomes very thick and heavy, about 1-2 minutes.

Remove from heat. Immediately whisk in the vanilla extract and the pieces of butter until the butter is melted and the custard is smooth and glossy.

Place one cooled pastry rectangle in a 9x13 inch dish or on a cutting board. Pour the hot custard over the base and spread evenly.

Top with the second pastry rectangle, pressing down gently. Cover with plastic wrap, pressing it directly onto the pastry surface.

Refrigerate for at least 4-6 hours, or overnight, until the custard is completely firm and set.

Once chilled, use a serrated knife to cut into squares. Dust generously with powdered sugar (and cinnamon, if using) and top with fresh raspberries before serving.