Preheat the oven to 350° Fahrenheit and grease an 8-inch square pan with olive or avocado oil cooking spray.
Add all your dry ingredients (almond flour, protein powder, sweetener, baking powder, salt, cinnamon, and nutmeg) into a large mixing bowl. Using either a fork or a whisk mix the ingredients together until everything has been evenly distributed.
Next, add your eggs, melted butter, almond milk, vanilla, and apple flavoring to the mixture. Whisk together, until you have a nice smooth batter and everything has been well incorporated.
Add the diced apples (if using) and stir again until they have been evenly distributed throughout the batter.
Pour your cake batter into your prepared pan and use a spatula to spread it evenly in the pan.
Make the streusel by combining all three ingredients in a small bowl, and mix together.
Sprinkle the streusel topping over the cake batter and bake for 35 to 40 minutes or until the edges have begun to brown and the middle no longer jiggles when lightly shaken.
Pull your cake out of the oven, and let it cool completely before cutting and serving. Cut it into nine equal slices, and enjoy!