Preheat the oven to 325ºF and grease a 9x9 inch metal baking pan.

For the topping, whisk 3/4 cup almond flour, 1/3 cup brown sugar replacement, 1 1/2 tbsp coconut flour, and 1/4 tsp salt together in a medium bowl. Stir in 1/4 cup melted butter until the mixture clumps together. Set aside.

In a large bowl, combine 1 1/2 cup almond flour, 1/2 cup erythritol sweetener, 1/4 cup whey protein powder, 2 tbsp coconut flour, 1 1/2 tbsp baking powder, and 1/4 tsp salt. Add 2 large eggs, 1 tsp vanilla extract, and 1/4 cup melted butter, then stir in 1/2 cup water until the batter is well mixed.

Spread the batter in the prepared pan and sprinkle with 1 cup chopped peaches. Then sprinkle the crumb mixture over top.

Bake for 40 to 45 minutes, or until the crumb is nicely browned and the cake is just firm to the touch. Remove and let cool completely before cutting into 16 pieces.