Mix the flour, ground almonds, cornstarch, baking powder and sugar. Pile the mixture on a work surface. Cut the butter into pieces and place around it. Add the butter and almond extract. Chop the dough with a large knife. Knead quickly into a dry dough. Add 1 tablespoon of water as needed. Form a ball and wrap in plastic. Refrigerate for at least 30 minutes.
Preheat the oven to 180°C (approximately 350°F). Line a baking sheet with parchment paper.
Cut the dough into 20-25 pieces. Form small balls and set 2 inches apart on the baking sheet. Press a whole almond into the center of each. Bake for 10-15 minutes. Cool on a wire rack before serving.