In a bowl, mix together first 5 ingredients until combined. Taste and adjust sweetener as needed. Set aside.
In a separate bowl whisk protein powder and flaxseeds, then stir in milk.
Pour the wet ingredients into the protein powder mixture and stir until combined.
Line an 8 by 8 baking dish with parchment paper.
Press batter onto parchment paper and level evenly as much as possible.
If using walnuts press them into the top of the batter.
Refrigerate 1 hour then holding the edges of the parchment paper, lift onto a cutting board.
Slice into 12 bars and keep refrigerated.