When ready to bake, place cupcake liners in a 12-cup muffin pan and preheat the oven to 425°F.
Make the streusel by mixing oat flour*, all purpose flour, white sugar, brown sugar, and salt. Add in softened butter and combine until the mixture is coarse and resembles small pebbles. Set aside.
Spoon batter into the muffin pan equally to make 12 muffins.
Sprinkle 2-3 tablespoons of streusel over each muffin.
Place the muffin pan in the oven and bake for 10 minutes.
Without opening the oven, reduce the temperature to 350°F and bake for an additional 8 minutes.
Test the center of a muffin with a toothpick. They are done once it comes out clean or with a few crumbs.
Carefully remove each muffin and place on a cooling rack.