01 - Preheat oven to 350°F. Line a baking sheet with parchment paper.
02 - Whisk together flour, baking powder, baking soda, and salt in a bowl.
03 - Beat softened butter and granulated sugar until light and fluffy. Add egg and vanilla extract, mixing well.
04 - Gradually incorporate dry ingredients into the wet mixture, alternating with milk, until a soft dough forms.
05 - Roll tablespoon-sized portions of dough into balls. Place on prepared baking sheet 2 inches apart, flatten slightly. Bake for 10-12 minutes until edges are lightly golden. Cool completely on wire rack.
06 - Whisk together milk, sugar, egg yolk, cornstarch, vanilla, and salt in a saucepan. Cook over medium heat, whisking constantly, until mixture thickens, approximately 3-4 minutes. Remove from heat, cover with plastic wrap directly touching surface, and refrigerate until set.
07 - Combine chopped chocolate, heavy cream, and butter in microwave-safe bowl. Microwave in 20-second intervals, stirring between each, until smooth and glossy. Allow to cool slightly.
08 - Pipe or spoon approximately 1 teaspoon of custard onto the flat side of half the cookies. Place remaining cookies on top to form sandwiches.
09 - Spoon or pipe chocolate glaze over the top of each cookie sandwich. Allow chocolate to firm completely before serving.