Whisk together the all-purpose flour, 3/4 cup protein powder, salt, and 1/4 cup sugar in a large bowl until evenly combined.
In a microwave-safe bowl, heat 3/4 cup milk with 3 tablespoons butter for 30–40 seconds until the butter softens; whisk until combined, then stir in the yeast.
Add the wet mixture and 1 large egg to the dry ingredients and mix until a soft dough forms.
Lightly flour a work surface and knead the dough for 2–3 minutes until smooth, then place it in a greased bowl and let rest for 5 minutes.
In a small bowl, combine 1/4 cup plus 1 tablespoon brown sugar and 1 tablespoon cinnamon.
Roll the dough on a floured surface into a 15 x 6-inch rectangle.
Spread the softened 3 tablespoons butter over the dough, leaving a 1/2-inch border, then evenly sprinkle the cinnamon-sugar mixture on top.
Tightly roll the dough lengthwise into a log and slice into 12 equal pieces using a knife or unflavored dental floss.
Place the rolls cut-side up in a greased cake pan or baking dish, cover with a dish towel, and let them rise in a warm spot until doubled in size, about 45 minutes.
Preheat the oven to 350°F (180°C). Bake the risen rolls for 25 minutes or until golden at the edges.
While the rolls bake, make the frosting by whisking together 2/3 cup vanilla protein powder, 6 ounces softened cream cheese, and 1/4 cup milk until thick and fluffy.
Let the rolls cool for a few minutes, then spread the protein frosting over them and serve warm.
