Prepare a work surface with a silicone baking mat or a large piece of parchment paper. Dust lightly with coconut flour.
In a medium bowl, whisk together the almond flour, hazelnut flour, coconut flour, and salt. In a large bowl, beat the butter and sweetener together until creamy. Beat in the egg and vanilla extract, and then beat in the almond flour mixture until dough comes together.
Turn the dough out onto the prepared work surface. Pat into a rough circle and then top with a large piece of parchment paper. Roll out to about 1/4-inch thickness. Line two large baking sheets with silicone mats or parchment paper.
Using a 2 1/2 inch round cookie cutter, cut out as many cookies as possible and lift carefully with a small, offset spatula or knife. Collect the scraps and re-roll, cutting out as many circles as possible. You should get at least 32 cookies.
Use a smaller 1 inch cookie cutter in any shape to cut out windows in half of the cookies. (Try keeping all the bottom cookies on one cookie sheet and all the tops with the windows on the other, as they will bake at slightly different rates).
Chill the cut outs for 1 hour. Then preheat the oven to 325F and bake the cookies for 15 to 20 minutes, rotating and switching the pans halfway through baking, until lightly golden brown. Remove and let cool completely on the pans.
While the cookies are cooling, make the raspberry filling. In a medium saucepan over medium heat, combine the raspberries and water. Bring to a boil, then reduce the heat to a simmer. Cook until the berries can be mashed easily with a fork.
Remove from heat and stir in the powdered sweetener, then whisk in the gelatin. Let cool until thickened but still spreadable, about 20 minutes.
Sprinkle the tops of the cookies with the window cut out with powdered sweetener.
Spoon about 2 teaspoons of raspberry filling onto the bottom of the cookies with no center cut out. Gently press a cookie top so that the raspberry filling spreads and pushes up a bit through the window of the top cookie.
If you want to use regular store-bought gelatin like Knox, use about 1 envelope. You will need to "bloom" it first by whisking it into 2 tbsp of warm water. Once it gels, whisk it into the hot raspberry mixture (off heat) until it dissolves.