Step 1: Prepare the Vanilla Cookies. In a large bowl, cream together softened butter and granulated sugar until light and fluffy. Beat in the egg and vanilla extract until well combined.
Step 2: In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. If the dough is too stiff, add 1-2 tablespoons of milk.
Step 3: Roll the dough into small, 1-inch balls and place them 2 inches apart on baking sheets lined with parchment paper. Slightly flatten each ball. Chill the dough balls for at least 30 minutes.
Step 4: Preheat your oven to 350°F (175°C). Bake the chilled cookies for 8-10 minutes, or until the edges are lightly golden and the centers are set. Let cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
Step 5: While cookies cool, prepare the Pastry Cream. In a medium saucepan, heat 1 3/4 cups of milk over medium heat until simmering. In a separate bowl, whisk together egg yolks, granulated sugar, cornstarch, and salt until smooth. Whisk in the remaining 1/4 cup of cold milk.
Step 6: Slowly temper the egg yolk mixture by whisking about half of the hot milk into it. Pour the tempered mixture back into the saucepan with the remaining hot milk, whisking constantly.
Step 7: Cook over medium heat, whisking constantly, until the mixture thickens considerably and comes to a boil. Boil for 1 minute, still whisking. Remove from heat and stir in the cold butter and vanilla extract until smooth.
Step 8: Transfer the pastry cream to a clean bowl. Cover the surface directly with plastic wrap to prevent a skin from forming. Refrigerate for at least 2 hours, or until thoroughly chilled and set.
Step 9: Prepare the Chocolate Ganache. Place chocolate chips in a heatproof bowl. Heat heavy cream in a small saucepan or microwave until simmering. Pour the hot cream over the chocolate chips. Let sit for 5 minutes, then whisk until smooth and glossy.
Step 10: Assemble the Cookie Bites. Once the cookies and pastry cream are completely cool, whisk the chilled pastry cream briefly to smooth it out. Transfer the pastry cream to a piping bag with a round tip (or a zip-top bag with the corner snipped off).
Step 11: Pipe a generous dollop of pastry cream onto the flat side of half of the cookies. Top with the remaining cookies, gently pressing to create a sandwich.
Step 12: Dip the top of each cookie bite into the chocolate ganache, allowing any excess to drip off. Place on a wire rack to set. For a quicker set, refrigerate for 15-20 minutes. Serve immediately or store in the refrigerator.