01 - Preheat oven to 350°F and line a baking sheet with parchment paper.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl.
03 - Beat softened butter and granulated sugar until light and fluffy. Incorporate egg and vanilla extract.
04 - Gradually blend dry ingredients into wet mixture, alternating with milk, until soft dough forms.
05 - Portion dough into 1-tablespoon balls, roll, and place 2 inches apart on prepared sheet. Flatten slightly with palm.
06 - Bake for 10 to 12 minutes until edges are lightly golden. Cool completely on wire rack.
07 - Whisk milk, sugar, egg yolk, cornstarch, vanilla, and salt in saucepan. Cook over medium heat, stirring constantly, until thickened, 3 to 4 minutes. Cover surface with plastic wrap and refrigerate until set.
08 - Combine chocolate, cream, and butter in microwave-safe bowl. Microwave in 20-second intervals, stirring until smooth and glossy. Cool slightly.
09 - Pipe or spread 1 teaspoon custard onto flat side of half the cookies. Top with remaining cookies to form sandwiches.
10 - Spoon or pipe chocolate glaze over top of each cookie sandwich. Allow chocolate to set until firm.