Preheat oven to 350°F.
Cream together butter and 1 cup sugar until light and fluffy in a large bowl.
Add egg and 1 teaspoon vanilla extract, mixing until fully incorporated.
In a separate bowl, whisk flour, baking powder, and salt; gradually add to butter mixture until dough forms.
Roll dough to 1/4 inch thickness on a lightly floured surface and cut into rounds.
Place rounds on baking sheet and bake 8-10 minutes until edges are lightly golden. Cool completely on wire rack.
For the pastry cream, heat milk in a saucepan until just simmering.
In a bowl, whisk egg yolks, 1/4 cup sugar, and cornstarch until smooth.
Slowly whisk hot milk into yolk mixture, then return to pan and cook over medium heat, whisking constantly, until thickened.
Remove from heat and stir in 1 teaspoon vanilla extract. Cool completely.
For the chocolate glaze, heat heavy cream until steaming and pour over chocolate chips. Let sit one minute, then stir to smooth.
Assemble cookies by spreading about 1 teaspoon pastry cream on bottom of one cookie, topping with a second cookie.
Dip the top of each sandwich in chocolate glaze and allow glaze to set before serving.