Prep the pan and oven: Heat the oven to 350°F (175°C). Line an 8-inch square pan with parchment, leaving flaps for easy lifting.
Lightly grease.
Make the crumb topping: In a bowl, mix 1 cup almond flour, 1/3 cup granulated sweetener, 1 teaspoon cinnamon, a pinch of salt, and 1/2 cup chopped pecans. Cut in 4 tablespoons cold butter until crumbly. Chill while you make the batter.
Whisk dry ingredients: In a large bowl, combine 2 cups almond flour, 2 tablespoons coconut flour, 1/2 cup granulated sweetener, 2 teaspoons baking powder, 1 tablespoon pumpkin pie spice, 1/2 teaspoon cinnamon, and 1/4 teaspoon salt.
Mix wet ingredients: In another bowl, whisk 3 large eggs, 3/4 cup pumpkin puree, 1/3 cup sour cream (or Greek yogurt), 1/4 cup melted butter, and 2 teaspoons vanilla.
Combine: Stir the wet mixture into the dry until fully incorporated.
The batter will be thick but spreadable. If it’s too thick, add 1–2 tablespoons almond milk.
Assemble: Spread the batter evenly in the prepared pan. Sprinkle the chilled crumb topping over the surface, pressing lightly so it adheres.
Bake: Bake for 28–35 minutes, until the edges are set and a toothpick inserted in the center comes out mostly clean with a few moist crumbs.
Do not overbake.
Cool: Let the cake cool in the pan for at least 20–30 minutes before lifting out. It sets as it cools, which improves the texture.
Optional glaze: Stir 1/3 cup powdered sweetener with 1–2 tablespoons cream or almond milk and a splash of vanilla. Drizzle over the cooled cake.
Slice and serve: Cut into 9 large or 12 small squares.
Enjoy warm or at room temperature.