Mix together the beef gelatin and boiling water in a small bowl and stir to dissolve; set aside.
Cream together the butter, sour cream, Swerve Confectioners, egg, vanilla extract, almond extract, and stevia glycerite in a large bowl and set aside. Beat in the dissolved gelatin.
Whisk together the almond flour, coconut flour, and baking powder in a medium bowl.
Melt the mozzarella cheese in a microwave (in 30 second increments) or in a double boiler.
Add the almond flour and mozzarella cheese to the butter mixture and stir vigorously to combine. It might be helpful to knead the dough a bit with oiled hands to help it come together.
Form the dough into a ball, flatten it slightly into a disk, wrap it in plastic wrap, and refrigerate until chilled (about 2 hours in the fridge or 20 minutes in the freezer).
Preheat the oven to 350F and line 2 large baking sheets with parchment paper or silpat liners. Roll the cookie dough out between 2 pieces of parchment paper to a circle about 1/4-inch thick. Use a cookie cutter in any shape you like to stamp out the dough into cookies, and then gather the scraps, re-roll, and repeat until the dough is all used. I used a 3-inch circle cookie cutter and got 12 cookies.
Arrange the cookies on the prepared baking sheets and bake until golden on the bottom, about 12 to 15 minutes, rotating the trays once halfway through. Cool completely before frosting.
Add the cream, egg yolks, Swerve Confectioners, stevia glycerite, and salt to a small saucepan over medium heat. Cook until the mixture is thickened, about 3 minutes, whisking constantly. Cool to room temperature and then chill.
Use a handheld electric mixer to cream the butter and vanilla together in a large bowl. Beat in the chilled cream mixture until light and fluffy.