In a large bowl, melt butter and stir in the heavy whipping cream.
Add the cocoa powder, coffee or espresso powder, vanilla and sweetener. Whisk until smooth.
Add the eggs, one by one, mixing very well between each.
Sieve in the flours: lupin, whey protein and flax meal, plus the baking powder, baking soda and xanthan gum. Mix until the flours are well incorporated.
Add warm water, slowly, while whisking. Let the batter sit for a couple of minutes, and give it a last mix before filling the liners.
Pour the batter into 12 silicone cupcake liners or directly into the buttered holes of a muffin pan.
Take the tray into a preheated oven at 170 °C (340 °F) for 25 minutes, or until a toothpick comes out mostly clean.
Peanut butter protein frosting
Add water to whey, little by little, and mix until creamy and completely lump free
Add peanut butter and sweetener drops, and vanilla if using it
Mix vigorously until it has a hard caramel/soft toffee like consistency. Try it and adjust sweetener if necessary.
If it's too hard to pipe, add more water. If it's too soft to pipe, add more whey
Notes
I add stevia (or monk fruit) powder to my erythritol to make it 100% as sweet as sugar.This way, I can use less erythritol, avoiding the cooling taste that shows up when it's used in excess, while also masking the bitter aftertaste of stevia and bulking it up. There are many brands that sell this sweetener concoction prepacked, either as erythritol plus stevia, or erythritol plus monk fruit, or erythritol plus both stevia and monk fruit. I just find it cheaper to make my own 😉