Heat your oven to 180 Celsius (350 Fahrenheit) and line a 9x5-inch loaf pan with parchment paper. Ensure the parchment paper extends slightly over the pan sides to create convenient handles for lifting the baked loaf later.

Whip the whole eggs with an electric mixer until they become pale, frothy, and double in total volume, then blend in the primary portion of granulated sweetener for another minute. The beaten egg mixture will look light and aerated with tiny bubbles suspended throughout the liquid.

Pour the melted butter, pure vanilla extract, and unsweetened almond milk into the whipped eggs and blend at low speed. The wet mixture will turn smooth and light golden with a fragrant vanilla aroma.

Whisk the almond flour, coconut flour, unflavored whey protein powder, and baking powder together in a separate bowl, then combine them thoroughly with the wet mixture. The combined batter will develop a thick, spreadable consistency that holds soft peaks when lifted with a spatula.

Mix the remaining granulated sweetener and ground cinnamon together in a small bowl to assemble the spiced ribbon filling. The cinnamon blend will look evenly tinted with a fine, uniform granular texture.

Spread half of the prepared batter across the bottom of the lined loaf pan, dust half of the cinnamon mixture over it, cover with the remaining batter, and finish with the rest of the spice mix on top. Gently tap the filled pan flat on the counter several times so the batter settles firmly without hidden air gaps.

Glide a table knife through the batter in an up-and-down and side-to-side pattern to distribute the spice ribbon, tap the pan once more, and bake for 50 to 55 minutes until an inserted skewer emerges clean. The crust will turn rich golden brown while releasing a comforting cinnamon perfume throughout your kitchen.

Inspect the loaf at the 40-minute mark and tent a sheet of aluminum foil loosely over the top if the crust has already achieved deep browning. The loose foil shield deflects direct top heat while allowing the interior crumb to finish setting completely.

Remove the loaf from the oven and allow it to cool completely in the pan before attempting to lift or slice. The interior crumb structure firms up during cooling, ensuring neat and clean slices that hold together perfectly.