Prep your pan and oven: Heat the oven to 350°F (175°C). Line an 8-inch square or 9x5-inch loaf pan with parchment and lightly grease the sides.

Mix the dry ingredients: In a large bowl, whisk almond flour, oat flour, whey protein, sugar, sweetener (if using), baking powder, baking soda, and salt until well combined. Break up any clumps of protein powder.

Combine the wet ingredients: In a separate bowl, whisk eggs, Greek yogurt, milk, oil, and vanilla until smooth and glossy. The yogurt should be fully blended with no streaks.

Bring it together: Add the wet mixture to the dry. Stir with a spatula until just combined. The batter will be thick but spreadable. Do not overmix or the cake can turn dense.

Customize: Fold in chocolate chips or berries. For a marble effect, remove 1 cup of batter, stir in cocoa powder, then swirl it into the pan with the plain batter.

Bake: Spread the batter evenly in the pan. Bake for 28–36 minutes, depending on your pan and oven. The cake is done when the top is set, lightly golden, and a toothpick comes out with a few moist crumbs.

Cool properly: Let the cake cool in the pan for 10 minutes, then lift it out using the parchment and transfer to a rack. Cool for at least 30 minutes before slicing for the best texture.

Optional glaze: Stir 1/2 cup Greek yogurt with 1–2 tablespoons maple syrup or powdered sweetener and a splash of vanilla. Spread over the cooled cake.