✓ Preheat oven to 350°F (180°C). Line two baking sheets with parchment paper.

✓ In a large mixing bowl, combine the flour, cocoa powder, powdered sugar, and salt. Whisk until well blended.

✓ Add the cold, cubed butter to the dry ingredients and use your fingertips or a pastry blender to work the butter into the mixture until it resembles coarse crumbs.

✓ Mix in the egg yolk and 2 tablespoons of water. Stir just until the dough comes together; avoid overmixing as this can make the tartlet shells tough.

✓ Divide the dough into small, evenly sized pieces (about 1 tablespoon each). Roll each piece into a ball and then gently flatten to form discs about 1/4 inch thick.

✓ Place the discs onto the prepared baking sheets and bake for 10-12 minutes or until they start to set at the edges but remain soft in the center.

✓ Allow the tartlet shells to cool completely on a wire rack before filling.