Preheat your oven to 325°F. Grease a 9-inch square baking pan and line the bottom with parchment paper. In a large bowl, whisk together the almond flour, coconut flour, 3/4 cup erythritol, baking powder, and salt.
In a separate bowl, whisk the eggs, sour cream, melted butter, and vanilla extract until smooth. Pour into the dry ingredients and stir until a thick, smooth batter forms.
For the cinnamon swirl, mix together 1/3 cup erythritol, 2 tbsp cinnamon, and 2 tbsp melted butter until it forms a paste. For the crumb topping, combine the almond flour, 3 tbsp erythritol, and cinnamon in a small bowl. Cut in the cold butter with your fingers until you have coarse, pea-sized crumbs.
Spread half of the cake batter into the prepared pan. Dollop spoonfuls of the cinnamon swirl mixture over the batter and use a toothpick or skewer to swirl it through in a decorative pattern.
Spread the remaining batter on top and add any remaining cinnamon swirl, swirling again. Scatter the crumb topping evenly over the surface. Bake for 35 to 40 minutes until a toothpick inserted in the center comes out clean and the crumb topping is golden. Let it cool in the pan for at least 20 minutes.
While the cake cools, whisk together the powdered erythritol, heavy cream, and vanilla extract until smooth. Drizzle the glaze over the warm coffee cake in a zigzag pattern.
Let the glaze set for 5 minutes, then slice and serve warm. This cake is perfect alongside a hot cup of coffee.
