Add the cocoa powder, almond flour, casein protein, and a pinch of salt to a mixing bowl. Whisk thoroughly to break up any clumps, especially the cocoa, so the batter turns out smooth and evenly chocolatey.

In a separate bowl, combine the Greek yogurt, egg, sweetener, almond milk, and vanilla extract. Whisk until the mixture becomes completely smooth and creamy with no streaks. This helps the cake stay moist and gives it a rich texture.

Pour the dry mixture into the bowl of wet ingredients. Use a rubber spatula to fold everything together slowly. Stop mixing as soon as the batter looks glossy and uniform. Overmixing can make protein cakes tough, so gentle folding is key.

Line an 8×8-inch baking pan with parchment paper, letting it hang over the edges for easy lifting later. Spread the thick batter evenly into the pan, smoothing the top so it bakes uniformly.

Place the pan in a preheated 350°F (180°C) oven and bake for 18–24 minutes. Check the center — it should look set but still slightly soft for the best fudgy texture. Avoid overbaking, as it can make the cake dry.

Remove the pan from the oven and let the cake cool completely in the pan. Cooling allows the fudge texture to firm up and makes slicing clean and easy. Once cooled, lift it out using the parchment and slice into squares.