In a mixing bowl, combine 1 and 1/2 cups almond flour, 1/4 cup coconut flour, and 1/4 teaspoon salt. In another bowl, whisk together 1 large egg, 1/3 cup melted butter, and 1 tablespoon erythritol (or your preferred keto sweetener). Pour the wet ingredients into the dry ingredients and mix until it forms a dough. Press the dough into a 9-inch pie pan evenly across the bottom and up the sides. Prick the bottom with a fork to prevent bubbling.