Add almond flour, flaxseed meal, whey protein, baking powder, xanthan gum and salt to a medium bowl. Whisk until thoroughly combined, set aside. Note: feel free to make the flour mix ahead in a large batch and simply measure out 1/2 cup + 6 Tablespoons per bread.
Add egg and sweetener to a large bowl and beat with a whisk or electric mixer for 1 minute or two until airy and lighter in color. With the mixer on, add in the apple cider vinegar and butter.
Add in the dry flour mixture in two parts, alternating with the sour cream. Continue to mix for a minutes until fully incorporated and elastic, the batter will thicken as you mix (think almost cookie dough texture).
Transfer the batter to a microwave-safe dish or mold (use either a large one or divided into 3 ramekins), pressing down and smoothing out the top with a wet fingertip.
If you divide the dough in 3 you’ll cook the bread for 90 seconds on high, but if you cook it all together you’ll want to do 150 seconds (2-1/2 minutes). Time can vary a lot in the microwaves, but to give you an idea mine is 1000 watts. Either way, it’ll come out looking pale and spongy (and not very appetizing at all). But worry not, just wait for it to cool down until just lightly warm (it continues to cook as it cools). Then you’ll definitely want to give it a good toast to get some texture on.
Store in an airtight container for 2-3 days or in the freezer for a couple months. Serve warm with a pat of butter.