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 softened butter and granulated sugar until light and fluffy. Incorporate egg and vanilla extract.
04 - Gradually blend dry ingredients into the wet mixture, alternating with milk, until a soft dough forms.
05 - Scoop 1-tablespoon portions, roll into balls, and place on prepared baking sheet 2 inches apart. 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 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 each cookie sandwich. Allow chocolate to firm completely before serving.