Preheat oven to 375°F (190°C). Grease a standard 12-cup muffin tin and 2 additional cups in a second tin; set aside.
In a small bowl, mix together the flour, protein powder, pumpkin pie spice, cinnamon, baking powder, and sea salt. Set aside.
In a large bowl, whisk together the pumpkin puree and melted butter (or chosen substitute) until smooth.
Add the Splenda brown sugar, Splenda, and sugar (or vegan sweetener alternatives), whisking until combined and lump-free.
Stir in the eggs (or flax eggs) and vanilla extract until well incorporated.
Add the dry ingredients to the wet ingredients and stir until the batter is smooth.
Spoon about ¼ cup of batter into each muffin tin well and smooth the top.
Bake for 17-20 minutes, or until a toothpick inserted into the center comes out clean.
Let muffins cool in the pan for 10 minutes before transferring to a wire rack to cool completely.