Heat 1 cup (240ml) of whole milk in a medium saucepan until simmering. In a separate bowl, whisk together 3 egg yolks, 1/3 cup (67g) sugar, and 2 tbsp (16g) cornstarch until pale and smooth.

Slowly temper the egg mixture by whisking in about half of the hot milk. Pour the mixture back into the saucepan. Cook over medium heat, whisking constantly, until it boils and becomes very thick, about 1-2 minutes.

Remove from heat and whisk in 1 tbsp (14g) of butter and 1 tsp (5ml) of vanilla. Strain through a fine-mesh sieve into a bowl. Cover with plastic wrap pressed directly onto the surface and refrigerate for at least 2 hours until completely cold.

Preheat oven to 350°F (175°C) and line baking sheets with parchment paper. In a bowl, whisk together 1 1/2 cups (180g) flour, 1 tsp (4g) baking powder, and 1/4 tsp (1.5g) salt.

In a large bowl, cream 1/2 cup (113g) softened butter and 3/4 cup (150g) sugar until light and fluffy. Beat in the egg and 2 tsp (10ml) vanilla extract.

On low speed, alternate adding the dry ingredients and 1/4 cup (60ml) milk, mixing until just combined. Drop rounded tablespoons of dough onto the baking sheets.

Bake for 10-12 minutes, until edges are lightly golden-brown. Cool on the pan for 5 minutes before moving to a wire rack to cool completely.

Make the ganache: Place 4 oz (113g) of dark chocolate in a heatproof bowl. Heat 1/2 cup (120ml) of heavy cream until simmering, then pour over the chocolate. Let sit for 5 minutes, then whisk until smooth. Let cool for 10-15 minutes to thicken.

Assemble: Pipe or spoon the chilled pastry cream onto the flat side of half the cookies. Top with the remaining cookies to create sandwiches.

Glaze: Spoon the ganache over the top of each cookie sandwich, allowing it to drip down the sides. Let the ganache set for at least 30 minutes before serving.