Preheat oven to 350° F. Grease an 8-in springform pan, set aside.
In a mixer bowl fitted with the paddle attachment, mix together the almond flour, baking powder, salt, and sugar until well incorporated.
Add the cold, cubed butter and mix on low until a fine crumb forms. Add in the egg and vanilla, mix until combined. Mix in the sour cream just until combined.
For the filling, mix together the brown sugar and 1 teaspoon ground cinnamon. Set aside.
For the streusel, combine the melted butter, oats (oat flour), ⅛ teaspoon salt, ¼ cup granulated sugar, and the ½ tablespoon ground cinnamon. The mixture should be crumbly.
Pour half of the cake batter into the prepared pan, smooth out. Sprinkle the brown sugar, cinnamon filling one the batter. Swirl in with a butter knife. Pour the remaining half of the batter into the pan, smooth out. Evenly sprinkle the top of the cake with the oat streusel.
Bake for 35-40 minutes, or until a toothpick inserted into the center comes out clean.