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 - In a separate bowl, beat softened butter and granulated sugar until light and fluffy. Add egg and vanilla extract, mixing until incorporated.
04 - Gradually blend dry ingredients into butter mixture, alternating with milk, until soft dough forms.
05 - Roll dough into 1-tablespoon balls and arrange on prepared baking sheet 2 inches apart. Flatten gently with palm. Bake for 10-12 minutes until edges are lightly golden. Cool completely on wire rack.
06 - Whisk milk, sugar, egg yolk, cornstarch, vanilla, and salt in saucepan over medium heat. Stir constantly until mixture thickens, approximately 3-4 minutes. Remove from heat, press plastic wrap directly onto surface, and refrigerate until set.
07 - Combine chocolate, cream, and butter in microwave-safe bowl. Heat in 20-second intervals, stirring between each, until smooth and glossy. Allow to cool slightly.
08 - Spread or pipe approximately 1 teaspoon custard onto flat side of half the cookies. Place remaining cookies on top to form sandwiches.
09 - Drizzle or pipe chocolate glaze over each cookie sandwich. Allow glaze to firm completely before serving.