Preheat oven to 350 degrees Fahrenheit (175 degrees Celsius) and coat an 8x8 inch baking pan with non-stick spray.
Combine almond flour, cinnamon, melted coconut oil, and brown erythritol in a small bowl until the texture resembles coarse crumbs for the streusel.
In a large mixing bowl, whisk together the eggs, Greek yogurt, applesauce, and vanilla extract until smooth.
Sift the oat flour, vanilla whey protein, monk fruit sweetener, baking powder, and baking soda into the wet mixture.
Fold the dry ingredients into the wet ingredients using a spatula until just combined, ensuring no over-mixing.
Spread half of the batter into the prepared baking pan.
Evenly distribute half of the streusel mixture over the first layer of batter.
Layer the remaining batter on top and finish with the remaining streusel mixture.
Bake for 25 to 30 minutes, or until an inserted toothpick emerges clean.
Allow the cake to cool in the pan for 10 minutes before slicing into 8 equal portions.