Preheat the oven to 350ºF and grease an 8-inch round cake pan. Line with parchment paper and grease the paper.
In a large bowl, whisk together the sour cream, eggs, vanilla, and sweetener. Stir in the almond flour, protein powder, baking powder, cinnamon, and salt. If the batter is very thick, stir in water 1 tablespoon at a time. It should be thick but pourable.
Add the chocolate chips and stir until well distributed, then pour the batter into the prepared baking pan and spread to the edges.
Crumb Topping
In a medium bowl, whisk together the almond flour, sweetener, and butter until the mixture resembles coarse crumbs. Sprinkle all over the top of the cake.
Bake 40 to 50 minutes, until the top is just firm to the touch and the edges are golden brown. Remove and let cool completely in the pan.