Toast the Croissant Cubes: Preheat the oven to 375°F. Spread the croissant cubes on a baking sheet and toast until lightly golden and crisp. Let cool completely, then roughly chop again. Measure out 1 cup and set aside. You can use any leftover pieces for garnish. Lower the oven temperature to 350°F.
Make the Almond Filling: In a small bowl, mix the almond flour, sugar, beaten egg, melted butter, almond extract, vanilla extract, and salt until smooth. Roll the filling into teaspoon size balls. Chill the balls for 30 minutes.
Make the Cookie Dough: In a medium bowl, whisk together the flour, baking soda, baking powder, and salt. In a large bowl, beat the butter, brown sugar, and granulated sugar together until light and creamy. Add the eggs, vanilla extract, and almond extract and mix until smooth. Add the dry ingredients and mix on low until just combined. Gently fold in 1 cup of the toasted croissant pieces.
Assemble the Cookies: Scoop the cookie dough (about 1 1/2 tablespoons) and flatten slightly in your hand. Place a chilled almond filling ball in the center and wrap the dough around it, sealing completely. Gently roll into a ball.
Add Toppings: Roll the tops of the dough balls in sliced almonds, pressing lightly so they stick.
Bake: Place cookies on a lined baking sheet and bake until golden around the edges and set, about 10 to 12 minutes. Remove from the oven and let cool on the baking sheet for 5 minutes. If you have croissant pieces leftover, you can gently press some onto the tops of the cookies while they are still warm, if desired.
Finish: Once cooled, dust generously with confectioner’s sugar.