Prepare tray: Line a small baking pan or sheet with parchment, or use individual silicone snack molds.
Combine wet ingredients: In a large bowl, mash the bananas. Stir in the peanut or seed butter until smooth.
Add the oats, hemp hearts, protein powder (if using), and cocoa powder (if using). Mix until a thick, uniform dough forms. Add water 1 tablespoon at a time if using protein powder, only as needed, until it holds together. Taste and add maple or date syrup, one tablespoon at a time, as desired.
Press the mixture firmly into your tray or silicone molds. For perfectly even bars, place a sheet of parchment on top and press down with another tray or the bottom of a measuring cup to flatten the surface. Remove the top parchment.
Chill: Freeze for at least 3 hours (or refrigerate 3–4 hours) or until fully set.
For refrigerated tray oat bars: Slice after the mixture has chilled. Cold bars cut much more cleanly and hold their shape.
