Preheat oven to 350°F (175°C) and grease an 8-inch square baking pan.

Prepare the streusel by mixing rolled oats, melted coconut oil, maple syrup, and cinnamon in a small bowl until clumps form.

Whisk together flour, protein powder, baking powder, and salt in a large mixing bowl.

In a separate vessel, whisk eggs, sweetener, Greek yogurt, almond milk, and vanilla extract until fully emulsified.

Incorporate the dry ingredients into the wet mixture using a spatula, stirring until no dry pockets remain.

Transfer the batter to the prepared baking pan and level the surface with an offset spatula.

Distribute the oat streusel evenly across the top of the batter.

Bake for 30 to 35 minutes or until an internal temperature of 200°F is reached or a toothpick comes out clean.

Allow the cake to cool in the pan for at least 15 minutes before portioning.