Heat oven to 350ºF. Line a 9"x9" baking pan with parchment paper, letting the paper hang over the sides slightly.
Pour the oats and millet onto a rimmed baking sheet. Spread evenly and bake 8-10 minutes, or until lightly toasted. Remove from the oven and stir in the hemp hearts and flax seeds.
Add the coconut oil, brown sugar, and honey to a saucepan set over medium heat. Cook, stirring frequently, 2-3 minutes, or until melted. Stir in the cocoa powder, chocolate chips, chia seeds, and salt. Add the oat mixture, mixing well to fully combine.
Firmly press the granola into the prepared pan. Top with peppermint chips. Chill until set, about 2 hours in the refrigerator or 30 minutes in the freezer. Lift from the pan and remove the paper. Use a serrated knife to cut into 12 bars.