In a medium bowl, whisk the flour, baking powder, baking soda, and salt together. Set the bowl aside for now.
In a large bowl, cream the butter, brown sugar, and granulated sugar together with a wooden spoon. You can also use an electric hand mixer on high speed until fluffy, about 2 minutes.
Add in the egg yolks, vanilla, and almond extract and mix together with the wooden spoon (or on medium speed with an electric mixer until fluffy. Remember to scrape down the sides of the bowl as if needed).
Add the flour mixture and ground almonds to the wet ingredients and mix until combined. Be careful not to overmix.
Use a cookie scoop (2 tablespoons) to scoop the dough onto the prepared cookie sheets - 6 on each sheet. Make a slight indent in each dough ball and add ½ tablespoon of frangipane to the center of the dough, and flatten across the top of the cookie. Sprinkle the sliced almonds over the top.
Bake one cookie sheet at a time for 12 - 13 minutes, (mine take 13 minutes), until they are lightly golden brown.
Remove from the oven and let the cookies cool on the warm cookie sheet for 5 minutes then carefully transfer them with a spatula to a wire rack to cool completely.
Once the cookies have cooled, dust the tops with powdered sugar. (I put a tablespoon of sugar into a small sieve and shake it over the top of the cookies).