Combine dry ingredients. In a large bowl, mix oats, protein powder, cocoa powder, ground flaxseed, and salt.
Add wet ingredients. Add peanut butter, honey, and vanilla extract. Mix with a spoon or your hands until everything clumps together. If it's too dry and crumbly, add water or milk 1 tablespoon at a time.
Add chocolate chips if using, then fold them into the mixture.
Roll into balls using about 1 tablespoon of mixture per ball. Wetting your hands slightly prevents sticking. You should get about 24 balls total.
Chill on a parchment-lined plate for at least 30 minutes to firm up.
Store in an airtight container with parchment paper between layers. Refrigerate for up to 2 weeks or freeze for up to 3 months.