Preheat oven to 350℉. Line a baking sheet with parchment paper. Set aside.
Mix flour, baking powder, cornstarch and salt in a separate bowl. Set aside.
Using your standing mixer or a hand-held beater, cream butter and brown sugar together until light and fluffy. Add vanilla powder and stir well.
Add eggs and beat until well incorporated, scraping down sides of the bowl as needed with a rubber spatula. Stir in dry ingredients.
Using a cookie scoop or your hands, make 36-48 balls (depending on the size cookie you like) from the dough. Place 12 on the baking sheet at a time and bake for 10 minutes, or until the edges are golden-brown and the centers are set, but still soft. Remove from baking tray to a cooling rack.
These cookies store well in an air-tight container for several days.