Mix yogurt, stevia, and whey protein until smooth.
Fold in brownie pieces.
Freeze for a few hours, stirring every half hour or so. Or follow the instructions an your ice cream maker.
Scoop, and enjoy!
Notes
A note about substitutions: You could use your favorite chocolate vegan or casein protein in the yogurt. But because each type of protein powder has a distinct flavor and texture when baked, I would not try substituting different types of protein powders in the brownie. (See my Recipe Basics video for examples of what goes wrong when you use the wrong protein powder.)