Preheat oven to 350°F (175°C). Grease a baking dish.
Whisk almond flour and baking powder together.
In a separate bowl, whisk eggs, melted butter, granulated sugar substitute, sour cream, and vanilla.
Combine wet and dry ingredients until just combined.
Mix walnuts, brown sugar substitute, and cinnamon for the streusel.
Pour half the batter into the prepared dish. Sprinkle with half the streusel. Repeat with remaining batter and streusel.
Swirl gently with a knife.
Bake 35–40 minutes, until a toothpick inserted in the center comes out clean.
Cool at least 15 minutes before slicing.