Directions

1. Preheat oven to 350°F with a rack in lower third. Brush an 8-inch square baking pan with softened butter. Line bottom of pan with parchment paper, leaving an overhang on two sides.
2. Put almond flour, coconut flour, arrowroot flour, coconut sugar, and salt in the bowl of a food processor and pulse to combine. (If you don't have a food processor, just stir mixture together in a bowl.)
3. Cut cold butter into pieces. Add butter and egg to food processor. Pulse until mixture is crumbly and holds together when pinched. (If you don't have a food processor, cut in butter with a pastry blender or rub it with your fingers until crumbly, then add egg and stir thoroughly until dough is well blended and holds together.)