1. Make the Vanilla Cookies:

1. Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.
2. In a large mixing bowl, cream the softened butter and sugar until light and fluffy. Beat in the egg and vanilla extract.
3. In a separate bowl, whisk together the flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until a soft dough forms.
4. Scoop tablespoon-sized portions of dough onto the prepared baking sheets, spacing them about 2 inches apart. Flatten slightly with the back of a spoon or your fingers.
5. Bake for 8-10 minutes, or until the edges are lightly golden. Let cool completely on a wire rack.

2. Prepare the Pastry Cream Filling:

1. In a saucepan over medium heat, warm the milk until it begins to steam (do not boil). Remove from heat.
2. In a mixing bowl, whisk together the egg yolks, sugar, and cornstarch until pale and thick. Slowly whisk in half of the warm milk to temper the eggs.
3. Pour the egg mixture back into the saucepan with the remaining milk. Cook over medium heat, whisking constantly, until the mixture thickens and comes to a boil. Boil for 1 minute, then remove from heat.
4. Stir in the butter and vanilla extract until smooth. Transfer the pastry cream to a bowl, cover with plastic wrap (pressing it directly onto the surface to prevent a skin from forming), and chill in the refrigerator for at least 1 hour.

3. Assemble the Cookies:

1. Once cooled, pair up similar-sized cookies. Pipe or spread a generous dollop of chilled pastry cream onto the flat side of one cookie, then top with its matching cookie to create a sandwich.

4. Make the Chocolate Glaze:

1. In a microwave-safe bowl, combine the chocolate chips and heavy cream. Microwave in 20-second intervals, stirring between each, until smooth and glossy.
2. Dip the tops of the assembled cookies into the chocolate glaze, allowing any excess to drip off. Place on a wire rack and sprinkle with optional toppings like sprinkles or crushed nuts, if desired.

5. Chill and Serve:

1. Let the chocolate glaze set by chilling the cookies in the refrigerator for 15-20 minutes. Serve and enjoy these decadent Boston Cream Pie Cookies!