Creating the Boston Cream Pie Cookie Dessert is simple when you follow these steps:

1. Preheat the Oven: Begin by preheating your oven to 350°F (175°C). Line two baking sheets with parchment paper.
2. Prepare the Cookie Dough: In a bowl, whisk together the flour, baking soda, and salt. In a separate bowl, cream the softened butter, granulated sugar, and brown sugar until light and fluffy.
3. Add Eggs and Vanilla: Beat in the eggs one at a time, followed by the vanilla extract. Gradually add the dry ingredients to the wet mixture, mixing until just combined.
4. Scoop the Dough: Use a cookie scoop or tablespoon to portion out the dough onto the prepared baking sheets, spacing them about 2 inches apart.
5. Bake the Cookies: Bake in the preheated oven for 15-20 minutes or until the edges are lightly golden. Remove from the oven and let cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
6. Make the Custard Filling: In a saucepan, combine milk, sugar, and cornstarch over medium heat. Stir constantly until the mixture thickens. In a separate bowl, whisk egg yolks and slowly add a bit of the hot milk mixture to temper the eggs. Then pour the egg mixture back into the saucepan.
7. Cook the Custard: Cook for another 2-3 minutes, stirring constantly. Remove from heat, then stir in the vanilla extract and butter. Transfer to a bowl, cover with plastic wrap, and refrigerate until cooled.
8. Prepare the Ganache: In a small saucepan, heat the heavy cream until it begins to simmer. Pour it over the chocolate chips in a bowl and let it sit for 2-3 minutes. Stir until smooth and glossy.
9. Assemble the Cookies: Once the cookies are cool, spoon or pipe the custard filling onto the bottom of half of the cookies. Top with the remaining cookies to form sandwiches.
10. Finish with Ganache: Drizzle the chocolate ganache over the tops of the assembled cookies. Allow the ganache to set for a few minutes before serving.