Preheat the oven to 350°F. Grease an 8x8-inch baking pan and line it with parchment paper.
Make the crumb topping by mixing the almond flour, brown sugar substitute, cinnamon and salt. Rub in the cold butter with your fingertips until large, clumpy crumbs form, then refrigerate while you make the cake.
Blend the cottage cheese until smooth. Whisk it with the eggs, melted butter and vanilla in a large bowl.
In another bowl, whisk the almond flour, coconut flour, sweetener, baking powder and salt. Stir into the wet ingredients until a thick batter forms.
Spread the batter into the pan and scatter the crumb topping evenly over the top, pressing it in very gently.
Bake for 35 to 40 minutes, until the crumbs are golden and a toothpick in the center comes out clean. Tent with foil after 25 minutes if the topping browns too fast.
Cool in the pan for at least 30 minutes so the cake sets and the crumbs crisp up before slicing into squares.