Line a small baking sheet (9x13") with wax paper. Set aside.
Add peanut butter and white chocolate chips to a small pot and heat over medium-low heat. As chocolate chips begin to melt, stir continuously with a spatula until smooth and completely melted.
Slowly add casein powder to melted peanut butter and chocolate, whisking continuously. Once you add all the powder, it will get very thick and hard to stir. At this point, scoop the peanut butter mixture onto lined baking sheet.
Smooth down the mixture into about 1/4" thick rectangle. Lay another piece of wax paper on top of the mixture and smooth completely out. Place baking sheet into freezer for at least 30 minutes.
Line a larger baking sheet with wax paper. Arrange pretzels evenly on the baking sheet. Place dark chocolate chips in a microwave safe dish. Microwave for 30 seconds, then stir, then microwave for another 20 seconds and stir until melted and smooth.
Scoop chocolate into the corner of a plastic bag, then cut the very tip off the corner of the bag. Squeeze chocolate onto pretzels, then add sprinkles on top, if desired.
Add coconut oil to a 6-8 quart pot (one with a tight fitting lid) and melt over medium-high heat. Add a few popcorn kernels to the pot. Once they begin popping, add the rest of the popcorn kernels and cover the pot. As the popcorn begins popping, shake the pot to help keep the popcorn from burning. When popping slows (2-3 minutes), remove from heat and immediately pour popcorn onto a baking sheet and season with salt.
Remove peanut butter mixture from freezer and cut into chunks <1/2" square. Toss peanut butter chunks, pretzels, and popcorn before serving.