Prepare the Pastry Cream Filling:

• In a small saucepan, heat the milk over medium heat until warm but not boiling.
• In a separate bowl, whisk together the sugar, cornstarch, and egg yolks.
• Slowly pour the warm milk into the egg mixture while whisking constantly.
• Return the mixture to the saucepan and cook over low heat, stirring until thickened (about 3-4 minutes).
• Remove from heat, stir in vanilla extract and butter, then transfer to a bowl. Cover with plastic wrap (pressing it onto the surface to prevent a skin from forming) and refrigerate until cooled.

Make the Cookies:

• Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
• In a bowl, whisk together the flour, baking powder, and salt. Set aside.
• In a separate large bowl, cream together the butter, granulated sugar, and brown sugar until fluffy.
• Add the egg and vanilla extract, mixing well.
• Gradually add the dry ingredients, alternating with the milk, until fully combined.
• Scoop tablespoon-sized portions onto the prepared baking sheet, flattening slightly.
• Bake for 10-12 minutes, until the edges are lightly golden.
• Let cool completely on a wire rack.

Prepare the Chocolate Ganache:

• In a microwave-safe bowl, heat the heavy cream until hot but not boiling.
• Pour over the chocolate chips and let sit for 1-2 minutes, then stir until smooth.

Assemble the Cookies:

• Spoon or pipe a small amount of pastry cream onto the bottom of half of the cookies.
• Place another cookie on top, creating a sandwich.
• Spoon or drizzle the ganache over the tops of the cookies.

Chill & Serve:

• Let the cookies set for about 15-20 minutes before serving.
• Enjoy your homemade Boston Cream Pie Cookies!