Prepare the Vanilla Cookies

Preheat the oven to 350°F (175°C). Line a baking sheet with parchment paper.

Cream the softened butter and granulated sugar together until light and fluffy.

Add the egg and vanilla extract and mix until well combined.

In a separate bowl, whisk together the flour, baking powder, cornstarch, and salt.

Gradually add the dry ingredients to the wet ingredients. Mix just until a soft cookie dough forms.

Scoop the dough into evenly sized balls and arrange them on the prepared baking sheet. Gently flatten each ball.

Bake for 10 to 12 minutes, or until the edges are set and the centers remain soft.

Allow the cookies to cool for 5 minutes on the baking sheet before transferring them to a wire rack to cool completely.

Prepare the Vanilla Pastry Cream

Heat the milk in a saucepan over medium-low heat until hot but not boiling.

Whisk the egg yolks, sugar, and cornstarch together in a separate bowl until smooth.

Slowly whisk some of the warm milk into the egg mixture to temper the eggs. Gradually whisk the mixture back into the saucepan.

Cook over medium-low heat while whisking continuously until the pastry cream thickens.

Remove from the heat and stir in the vanilla extract and butter.

Transfer the pastry cream to a bowl, cover, and refrigerate until completely chilled.

Prepare the Chocolate Ganache

Place the finely chopped chocolate in a heat-safe bowl.

Heat the heavy cream until hot but not boiling, then pour it over the chocolate.

Allow it to sit for 2 minutes before stirring until smooth and glossy.

Let the ganache cool slightly until it reaches a spreadable consistency.

Assemble the Cookies

Spoon or pipe the chilled vanilla pastry cream onto each cooled cookie.

Spoon chocolate ganache over the pastry cream.

Refrigerate briefly to help the filling and ganache set before serving.