01 - Preheat oven to 350°F (175°C). Line two baking sheets with parchment paper and set aside.
02 - In a large mixing bowl, beat the softened butter and granulated sugar together using an electric mixer until pale, light, and fluffy. Add the egg and vanilla extract, mixing until fully incorporated.
03 - In a separate bowl, whisk together the all-purpose flour, baking powder, and salt. Gradually add the dry ingredients to the butter mixture, mixing on low speed until a soft dough forms. Cover and chill the dough while preparing the filling.
04 - Combine the finely ground almonds, granulated sugar, egg, softened butter, almond extract, and a pinch of salt in a small bowl. Stir until a thick, spreadable paste forms.
05 - Scoop approximately 1 tablespoon of dough and flatten it in the palm of your hand. Place ½ teaspoon of almond filling in the center, then fold the dough around the filling, shaping it into a crescent (croissant) form. Arrange on the prepared baking sheets.
06 - Gently press sliced almonds onto the surface of each shaped cookie, pressing lightly so they adhere to the dough.
07 - Bake for 13 to 15 minutes until the edges are golden brown. Allow the cookies to cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely.
08 - Once the cookies have cooled completely, dust generously with powdered sugar before serving.