Lightly grease a standard muffin tin.

In a medium-sized mixing bowl, cream together the butter, vegetable oil, and sugars. Add the eggs, beating to combine.

In a separate bowl, whisk together the oat flour, almond flour, baking powder, baking soda, nutmeg, and salt.

Beat the flour mixture into the butter mixture alternately with the milk, beginning and ending with the flour and making sure everything is thoroughly combined.

Preheat the oven to 425°F. Divide the batter into the 12 muffin cups, filling up each one almost to the top. Rest the batter while the oven heats.

Bake the muffins for about 18-20 minutes, until golden brown and a cake tester inserted into the middle of one of the center muffins comes out clean.

Remove the muffins from the oven, and let them cool for a couple of minutes, or until you can handle them. While they're cooling, melt the butter for the topping in the microwave or in a pot on the stove.

Use a pastry brush or your fingers to paint the top of each muffin with the butter, then sprinkle or roll the muffin in the cinnamon sugar.

Serve the muffins warm or at room temperature.