Optional: 1/4 cup more chocolate chipsmelted, for drizzling tops
Line a 8x8-inch pan with parchment or foil, leaving it to hang over two opposite edges (to use as handles to lift out and cut easier).
Add butter or coconut oil, honey, and vanilla to a large saucepan and melt together over medium heat. Mix in the oats and flax and cook, stirring constantly for a bit more (about 1-1/2 minutes). Remove from heat.
Press 1/2 to 2/3 of the mixture in the bottom of the prepared pan (it will be a thin crust), reserving 1/3-1/2 for the topping.
Melt chocolate and nut butter together over low heat until melted (or microwave 1 minute). Spread evenly over oatmeal crust.
Crumble reserved oatmeal mixture over the top and press it in a bit. Drizzle with melted chocolate chips, if desired.
Place in the fridge, covered, a couple hours until firm before cutting into 36 small bars.
Store in the refrigerator (they also freeze wonderfully).