Preheat oven to 350°F/180°C. Line a a muffin tray with paper liners (or butter and dust with coconut flour)
Add almond flour, flaxseed meal, whey protein, baking powder, xanthan gum and salt to a medium bowl. Whisk until thoroughly combined, set aside.
Add eggs and sweetener to a large bowl and beat with an electric mixer for 3-5 minutes 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 heavy cream. Continue to mix for a couple minutes until fully incorporated and elastic, the batter will thicken as you mix (think almost a cookie dough texture!).
Divide the batter into the muffin pan (using an ice cream scooper is ace here!). Smooth out the top with a wet spatula (or your fingertips).
Bake for 25-30 minutes, until golden and a toothpick inserted comes out clean. Watch out after minute 15, and tent with aluminum foil if they begin to brown too quickly (you know, an unfortunate tendency of allulose). The muffins will collapse a tad while cooling, don't sweat it (simply the fact of life with keto breads!).
Remove from the pan after 10 minutes and allow to cool completely before digging in for best texture.
Store in an airtight container for 3-4 days or in the freezer for a couple months. Serve warm with a pat of butter (non negotiable in my book!).
*How sweet? I like mine very lightly sweetened, so 1/4 cup does it for me (even if using allulose, which is 30% less sweet). But if you like yours on the sweeter side, even 1/2 cup will work (and the muffins will even have a tad bit more structure). Please note that nutrition facts were estimated per muffin.