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 until smooth.
04 - Gradually blend dry ingredients into wet mixture, alternating with milk, until soft dough forms.
05 - Roll dough into 1-tablespoon balls. Arrange on prepared baking sheet 2 inches apart. Flatten gently with palm.
06 - Bake 10 to 12 minutes until edges turn lightly golden. Transfer to wire rack and cool completely.
07 - Whisk milk, sugar, egg yolk, cornstarch, vanilla, and salt in saucepan. Cook over medium heat, stirring constantly, 3 to 4 minutes until thickened. Cover surface with plastic wrap and refrigerate until set.
08 - Place chocolate, cream, and butter in microwave-safe bowl. Microwave in 20-second intervals, stirring between each, until smooth and glossy. Cool slightly.
09 - Pipe or spread 1 teaspoon custard on 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 set until firm before serving.