In a small bowl, whisk together the melted butter, brown sugar replacement, and cinnamon until well combined.
Preheat the oven to 325F and grease a donut pan very well. This makes 8 regular donuts so grease 8 wells. If your pan only holds 6 wells, you may need to work in batches.
In a blender or food processor, combine the sour cream, eggs, sweetener, water, and vanilla extract and blend to combine. Add the almond flour, protein powder, baking powder, cinnamon, and salt and blend until smooth.
Spoon enough batter into the well of each donut cavity to cover the bottom. Drizzle a little of the cinnamon filling over each and then top with the remaining batter.
Bake 12 to 22 minutes, until golden brown and firm to the touch. Remove and let cool 15 minutes in the pan. Then transfer to a wire rack to cool completely.
Glaze and Decoration
In a small bowl, whisk together the powdered sweetener, cream, and lemon juice. Spread over the tops of the donuts. If your glaze is very thick, thin it out just a bit with a teaspoon or two of water.
Sprinkle in an alternating pattern with the three colors of sanding "sugar".