Heat the oven to 325°F. Grease and parchment-line an 8-inch square metal pan.
Whisk rolled oats, oat flour, flax, cinnamon, baking powder and salt in a large bowl. Add optional mix-ins.
Warm peanut butter, honey and brown sugar over low heat for 2 to 3 minutes, stirring just until glossy; do not boil. Remove from heat for 2 minutes.
Whisk egg, milk and vanilla separately, then stream into the warm peanut-butter mixture while whisking continuously.
Fold binder into dry ingredients until every oat is coated. Rest 5 minutes.
Squeeze a tablespoon. Add 1 tablespoon milk if it crumbles or 1 tablespoon oat flour if it smears, then wait 2 minutes.
Transfer to the pan, cover with parchment and compact to an even 3/4-inch layer using a flat-bottom measuring cup.
Bake 22 to 28 minutes until edges are light golden and the center is matte with gentle resistance.
Cool in the pan 30 minutes, lift to a rack and cool 45 to 60 minutes more. Chill 20 minutes if needed for cleaner cuts.
Cut the completely cool slab into four strips, then each strip into three bars using straight downward knife pressure.
