Preheat the oven to 350°F. Grease an 8x8 baking pan with non-stick spray.
In a large bowl, whisk together the gluten free flour, protein powder, Truvia monk fruit, baking powder, and salt. Add the Greek yogurt, applesauce, eggs, and vanilla. Mix until smooth, then fold in the sprinkles.
Pour the batter into the greased pan and bake for 20-23 minutes, until lightly golden and a toothpick comes out clean. Let it cool completely. (This is important. Warm cake will melt the frosting.)
While the cake cools, mix the Greek yogurt, Cool Whip, protein powder, pudding mix, Truvia monk fruit, vanilla, and salt in a medium bowl until smooth and creamy. Chill in the fridge until you're ready to assemble.
Use the rim of your glass or a small biscuit cutter to cut out circles the size of your glass. You want them to slide in snugly.
Place one cake circle in the bottom of your glass. Spread or pipe a thick layer of frosting on top. Add another cake circle. Top with a generous layer of frosting and smooth the top. Put in freezer for 15-20 minutes so frosting sets before adding sprinkles.
Pour rainbow nonpareils onto a small plate. Flip the glass upside down (carefully) and press the frosted top into the sprinkles until fully coated. Or just spoon them generously over the top if you don't want to flip.
Pop them in the fridge for 15 to 30 minutes to set, or eat immediately if you can't wait. (I never wait.)