1. Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.
2. In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
3. Beat in the eggs one at a time, then add the vanilla extract and mix well.
4. In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add the dry ingredients to the wet mixture, stirring until just combined.
5. Scoop tablespoon-sized portions of dough onto the baking sheets, spacing them about 2 inches apart.
6. Bake for 10-12 minutes or until the edges are lightly golden. Let the cookies cool completely on a wire rack.

2. Make the Custard Filling:

1. Heat the milk in a medium saucepan over medium heat until it begins to steam (but not boil).
2. In a separate bowl, whisk together the egg yolks, sugar, and cornstarch until smooth.
3. Gradually pour the hot milk into the egg mixture while whisking constantly to temper the eggs.
4. Return the mixture to the saucepan and cook over medium heat, whisking constantly, until the custard thickens (about 5 minutes).
5. Remove from heat, stir in the vanilla extract, and transfer the custard to a bowl. Cover with plastic wrap, pressing it directly onto the surface to prevent a skin from forming. Chill in the refrigerator until fully set.

3. Prepare the Ganache:

1. Place the chocolate chips in a heatproof bowl.
2. Heat the heavy cream in a small saucepan over medium heat until it starts to steam (but doesn’t boil).
3. Pour the hot cream over the chocolate chips and let it sit for 2-3 minutes. Stir until the ganache is smooth and glossy. Let it cool slightly before using.

4. Assemble the Cookies:

1. Spread a generous dollop of custard onto the flat side of one cookie. Top it with another cookie to create a sandwich.
2. Dip the top of each cookie sandwich into the chocolate ganache, allowing any excess to drip off. Place the cookies on a wire rack to set.
3. Once the ganache has firmed up, the cookies are ready to enjoy!