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 medium bowl.
03 - Beat butter and 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 - Scoop 1-tablespoon portions, roll into balls, and place 2 inches apart on prepared sheet. Flatten slightly with palm.
06 - Bake for 10–12 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, whisking constantly, until thickened, 3–4 minutes. Cover surface with plastic wrap and chill until set.
08 - Combine chocolate, cream, and butter in microwave-safe bowl. Microwave in 20-second bursts, stirring until smooth and glossy. Cool slightly.
09 - Pipe or spoon 1 teaspoon custard onto flat side of half the cookies. Top with remaining cookies.
10 - Spoon or pipe chocolate glaze over each cookie sandwich. Allow to set until chocolate is firm.