In a medium bowl, whisk together melted unsalted butter, granulated sugar, egg yolk, vanilla extract, almond extract, ground almonds, flour, and a pinch of salt. Mix until fully combined and smooth.

In a separate large bowl, whisk together all-purpose flour, baking powder, baking soda, and salt until blended. Cream together melted butter with the sugars until fluffy, about 3-4 minutes, then add in egg yolks, vanilla extract, and almond extract, mixing until well integrated.

Gently fold the flour mixture and ground almonds into the creamed mixture. Stir until just combined; avoid overmixing.

Using a cookie scoop, portion out the dough into balls, making an indent with your thumb and spooning about a teaspoon of frangipane into the center. Top with sliced almonds.

Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Arrange the cookies on the baking sheet, leaving space between them. Bake for 12-13.5 minutes until edges are golden and centers look set.

Once baked, let the cookies cool on the baking sheet for about 5 minutes, then transfer to a wire rack. Dust with powdered sugar before serving.