Preheat your oven to 325°F (160°C). Line two large baking sheets with parchment paper. Sift the almond flour, 1 cup of powdered sugar, and sea salt together in a bowl. Whisk to combine.

In a clean stand mixer bowl, whip the egg whites and cream of tartar on medium speed until foamy. Gradually add the granulated sugar one tablespoon at a time. Beat until stiff, glossy peaks form.

Using a silicone spatula, gently fold the sifted almond flour mixture into the egg whites in three additions. Fold just until the batter falls off the spatula in a thick, continuous ribbon.

Transfer batter to a piping bag with a 1/2-inch round tip. Pipe 1.5-inch discs onto the parchment paper. Tap the baking sheets firmly against the counter. Sprinkle half of the discs with the toasted sliced almonds.

Bake for 16 to 18 minutes until lightly golden-beige with a slightly cracked, textured surface. Let the discs cool completely on the baking sheet before removing.

Beat the softened butter on high speed for 3 minutes until creamy. Add the remaining 1 1/2 cups of powdered sugar, heavy cream, and toasted almond extract. Whip on high for 3-4 minutes until ultra-fluffy and off-white.

Transfer the cream to a piping bag with a large round tip. Pipe a thick layer onto the flat side of the non-sprinkled bottom meringue shells.

Cap with the matching almond-sprinkled top shells. Using a fine-mesh sifter, give the assembled French Almond Cookies a light dusting of fine white powdered sugar.