Preheat the oven to 350℉. Grease and line an 8x8-inch baking pan with parchment paper.
In a medium bowl, whisk together 1 ½ cups oat flour, 1 teaspoon baking powder, ¼ teaspoon baking soda, and ¼ teaspoon fine salt.
In a large bowl, whisk together the 6 tablespoons oil, 1 cup brown sugar, 1 teaspoon pure vanilla extract, 1 large egg, and ½ cup whole milk until smooth. Add the flour mixture and mix gently until just combined, then fold in the ¾ cup mini dark chocolate chips.
Pour the batter into the prepared and smooth the top. Rest for 20 minutes, then bake for 22-26 minutes until the top is set and a tester inserted in the center comes out clean
Notes
What type of pan? I like a simple, square cake, but feel free to bake this cake in a 9-inch greased and lined round cake pan. Or, double up and bake it in a 9x13-inch pan. Adjust baking times as necessary depending on the size of the pan.
Rest the batter. Oat flour really does need a bit of a rest to hydrate before baking. Try to let it rest for at least 20 minutes before sliding the pan into the oven.
Vegan. Substitute non-dairy milk for the milk, and use an egg substitute (like a flax egg).
Use cake flour. For a softer cake, try my gluten free cake flour (and use oat flour as the main flour, or whatever flour works best for you).
Add spices. For extra flavor and warmth, stir in 1 teaspoon ground cinnamon or ginger.
A few other add-ins. Throw in some nuts for crunch (chopped walnuts are lovely).