Prepare the Pan: Line a 9 x 5-inch loaf pan with parchment paper or lightly coat with cooking spray and set aside.
Process the Oats: In a food processor fitted with a steel blade, pulse the oats until finely ground, resembling oat flour. It’s fine if some larger pieces remain.
Combine Ingredients: Add the peanut butter, ground flaxseed, dates, molasses, honey, vanilla extract, cinnamon, protein powder, and salt to the food processor. Pulse to combine, adding 1 tablespoon of almond milk at a time until the mixture starts to come together. Adjust with more almond milk as necessary until the mixture is moldable and sticks together when pinched.
Add Optional Mix-Ins: Once the mixture is ready, remove it from the processor and stir in your choice of mix-ins (if desired).
Shape and Refrigerate: Transfer the mixture to the prepared loaf pan, pressing it evenly into the bottom. Refrigerate for at least 30 minutes to firm up.
Cut and Serve: Once set, cut the mixture into 8 bars (or squares, depending on your preference). Enjoy immediately, or store as instructed below.