Mix almond flour, coconut flour, and salt in a bowl.
In another bowl, beat butter and sweetener until creamy, then beat in the egg and vanilla. Add the dry mixture and stir until a dough forms.
On a surface lightly dusted with coconut flour, roll the dough to about 1/3-inch thickness between two sheets of parchment paper.
Cut into Halloween shapes (ghosts, pumpkins, bats) with cookie cutters. Place on lined baking sheets and freeze for 30 minutes.
Bake at 325°F for 12–14 minutes, until edges just start to brown. Cool completely on the pan.
For the icing, whisk together the sweeteners and egg white powder, then add water 1 tablespoon at a time until it drizzles in ribbons. Stir in vanilla and divide into bowls to color. Pipe outlines, let dry, then flood with icing and decorate with toothpicks. Let icing harden overnight.