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.
Recipes with the "Healthy Pick" stamp have been evaluated by a registered dietitian member of the Ordre professionnel des diététistes du Québec.