Preheat and prep the pan. Set your oven to 325°F (165°C). Use an ungreased 10-inch angel food cake tube pan with a removable bottom. Do not grease it; the batter needs the sides to climb.

Sift the dry blend. In a bowl, whisk together coconut flour and whey protein isolate. Sift them twice to remove any clumps. This step keeps the crumb ultra-light.

Get the egg whites ready. Add egg whites to a large, clean metal bowl. Make sure there’s no grease or yolk. Add cream of tartar and salt.

Start whipping. Using a hand mixer or stand mixer with a whisk attachment, beat the whites on medium speed until foamy and opaque.

Add sweetener gradually. Slowly sprinkle in the powdered sweetener, a tablespoon at a time, while beating. Increase to medium-high and continue until soft peaks form.

Flavor it. Add lemon zest, lemon juice, vanilla, and almond extract (if using). Beat just until incorporated. You still want soft to medium peaks, not stiff and dry.

Fold in the dry blend. Sift about one-third of the coconut flour/protein mixture over the whites. Gently fold with a spatula, sweeping from the bottom up. Repeat twice more, folding until no dry pockets remain. Keep it gentle to preserve air.

Fill the pan. Spoon the batter into the tube pan. Smooth the top with a spatula and gently run a knife through the batter to pop large air pockets.

Bake. Bake for 35–40 minutes, until the top is golden and springs back to a light touch. A skewer should come out mostly clean.

Cool upside down. Immediately invert the pan onto its built-in feet or over a bottle neck. Let it cool completely, 1–2 hours. This keeps it tall and fluffy.

Release and slice. Run a thin knife around the edge and center tube, then lift out the cake. Run the knife under the bottom to release. Slice with a serrated knife using a gentle sawing motion.

Serve. Dust lightly with powdered sweetener if you like. Add a few raspberries or a spoonful of sugar-free lemon curd for extra zing.