1. 1. Combine the dry ingredients: In a large mixing bowl, combine the rolled oats, protein powder, ground flaxseed, ground cinnamon, and salt. Whisk them together thoroughly until all the dry ingredients are evenly distributed.

2. 2. Warm the wet ingredients: In a separate microwave-safe bowl or a small saucepan over low heat, gently warm the nut butter, honey (or maple syrup), milk, and vanilla extract. You want it warm enough to be easily stirrable and pourable, but not boiling.

3. 3. Mix wet and dry: Pour the warm wet ingredient mixture into the bowl with the dry ingredients. Use a sturdy spoon or spatula to mix everything together. Keep mixing until all the dry ingredients are fully moistened and a thick, cohesive dough forms.

4. 4. Add optional mix-ins: If you’re using mini chocolate chips or chopped nuts, fold them into the mixture now. Distribute them evenly throughout the dough.

5. 5. Press into a pan: Line an 8×8 inch square baking pan with parchment paper, leaving an overhang on the sides. Transfer the protein bar mixture into the prepared pan.

6. 6. Firmly press and chill: Using the back of a spoon, your hands, or another piece of parchment paper, press the mixture down very firmly and evenly into the pan. Cover the pan and refrigerate for at least 30 minutes, or until the bars are firm.

7. 7. Slice and serve: Once thoroughly chilled and firm, use the parchment paper overhang to lift the entire slab out of the pan. Place it on a cutting board and slice it into 10-12 bars or squares, depending on your desired size.