- PREHEAT oven to 400F.
- STIR flour with baking powder and salt in a medium bowl. Set aside.
- BEAT butter with sugar in a large bowl, using an electric mixer on medium-high, until fluffy. Beat in egg and vanilla. Gradually beat in flour mixture, just until combined. Fill the mould with bits of dough, pushing down with your fingers to get rid of any air pockets. Flatten the top. (This batter will make exactly 8 cookie cups.) Set the mould on a baking sheet.
- BAKE in the centre of oven until cookies have puffed over the top and are dark golden, 30 to 35 min. If cookies are getting too dark during baking, cover with foil. Remove from oven and place the hot silicone mould on a flat surface. Place the baking sheet on top of the silicone mould to gently push down the batter back into the mould (the cookies will be soft). Let stand until cookies are slightly cool, then transfer to freezer and chill until cookies are completely cool.
- REMOVE cookie cups from mould. (This can be tricky. Luckily, the moulds are flexible so you can do some creative shimmy-ing to get the cups out.)
- BRUSH melted chocolate on the inside of cooled cups. Return to fridge until chocolate is firm, about 5 min. Fill with milk. Enjoy!