Use 2 large baking trays. Line with baking paper. Place flour, baking powder, butter, sugar, egg yolk, milk, mixed spice and vanilla in a bowl; knead into a dough. Shape into a ball and wrap in cling wrap; chill for 30 minutes.
Preheat oven to 180 degrees C. Sprinkle baking paper with flaked almonds.
Lightly flour work surface. Roll out pastry to 5mm thick; cut into pieces to fit figures in the spekulatius moulds. Dust moulds with cornflour and press the pastry into them.
Turn moulds over and strike so that the pastry shapes fall out. Place on baking trays. Remove any remaining cornflour by brushing the pastry with a little water.
Bake 10–15 minutes (this will depend on size and thickness) or until golden. Remove from oven. Cool on a wire rack.
Look in specialty cookware stores for rolling pins and moulds carved with festive scenes and designs. Windmills are the traditional Dutch design.