- In a bowl, combine the flour and baking powder. Set aside.
- In another bowl, cream the butter and sugar with an electric mixer. Add the egg yolk and vanilla and beat until smooth. Using a wooden spoon, stir in the dry ingredients. Shape the dough into a disk. Wrap in plastic wrap and refrigerate for about 30 minutes.
- With the rack in the middle position, preheat the oven to 180°C (350°F). Line a baking sheet with parchment paper.
- On a floured surface, roll the dough to a thickness of about 3 mm (1/8 inch). Using a 6-cm (2 1/2-inch) round cookie cutter, cut out 18 cookies. Using a thin spatula, lift each cookie onto the baking sheet.
- Bake until the cookies begin to colour, about 12 minutes. Let cool completely on the cookie sheet before handling.
- For each sandwich cookie, spread 15 ml (1 tablespoon) dulce de leche on the underside of a cookie. Top with a cookie. Spread with dulce de leche. Top with a third cookie, underside facing down. Spread dulce de leche around the sides of the sandwich and roll in the coconut. Repeat with the remaining ingredients. Store at room temperature.