Line an 8x8-inch baking pan with parchment paper (or plastic wrap) and set aside (adjust number/size of pan if altering batch size).
Pop* your amaranth by heating a large pot over medium-high heat. You'll know it's ready to start adding amaranth when a drop of water disperses quickly and balls up.
Add about 2-3 Tbsp amaranth at a time and immediately cover. Shake the pot back and forth over the heat (wearing oven mitts is helpful) to move the grain around. It should start popping within 1-3 seconds, and be done popping at the 10-second mark. Be sure to pull them off at around 10 seconds or they'll start to burn.
Not every single grain will pop (that's OK!). Very quickly empty into a mixing bowl. This may take a couple tries to dial in your heat and not burn them - it took me 4 tries! But then you'll hit your stride.
Continue until you have roughly 1 1/2 cups popped grain (amount as original recipe is written // adjust if altering batch size). As you go, add your popped grain into a small mixing bowl first to ensure none are burnt. Then transfer to a larger bowl so you don't mix any burnt grain with perfectly popped grain. Set aside.
Add peanut or almond butter and maple syrup to a medium mixing bowl and stir to combine. Then add protein powder and stir.
Add popped amaranth a little at a time until you have a loose "dough" texture. Be careful not to add too much or the bars can lose their tackiness and won't stick together. I found roughly 1 1/4 cups to be about right (amount as original recipe is written // adjust if altering batch size). Stir with a wooden spoon or use hands to disperse mixture evenly.
Transfer the mixture to the baking dish and press down to form an even layer. Lay parchment paper or plastic wrap on top and use a flat-bottom object like a liquid measuring cup to press down and pack the mixture into an even, firmly packed layer.
Transfer to freezer to set for 10-15 minutes or until firm to the touch. Then lift out and slice into 9 bars (amount as original recipe is written // adjust if altering batch size). Enjoy as is, or drizzle with a bit of melted dark chocolate (optional).
These get a little soft at room temperature, so store in the refrigerator (up to 5 days) or in the freezer (up to 1 month).
*Popping method from Craftsy.com. If you can't find amaranth, you can also sub puffed brown rice cereal or most other popped grains. *My go-to protein powder recently has been Garden of Life Raw Protein Powder in Vanilla, which works great, here! Other options would be Vega, Vegan Smart, or Sun Warrior. *I highly recommend making a big batch and freezing these! They'll stay for at least 1 month, likely longer! *Nutrition information is a rough estimate.