Preheat oven to 350°F (175°C). Lightly grease a mini muffin tin or line with mini paper liners.
In a large bowl, whisk together oat flour, cocoa powder, granulated sugar, brown sugar, baking powder, and salt until no lumps remain.
In a separate bowl, whisk eggs, oil, and vanilla until smooth.
Pour into the dry ingredients.
Stir with a spatula until just combined. Add 2 tablespoons milk to loosen the batter. If it’s still very thick, add up to 1 more tablespoon.
The batter should be thick but scoopable.
Fold in chocolate chips, if using.
Scoop batter into the mini muffin wells, filling each about three-quarters full. Sprinkle a pinch of flaky salt on top if you like.
Bake for 9–12 minutes, or until the tops are set and a toothpick comes out with moist crumbs (not wet batter). Err on the fudgy side.
Cool in the pan for 5 minutes, then transfer to a wire rack to finish cooling.
They firm up as they cool.