Preheat oven to 425 degrees F. Lightly grease standard muffin tin, or line tin with paper muffin cups.
In a stand mixer or medium bowl, combine butter, oil and sugar until smooth.
Add the eggs one at a time, beating after each addition. Add vanilla extract and combine.
In a small bowl, sift baking powder, baking soda, salt, nutmeg, gf flour blend, and guar gum.
Alternate adding part of the flour mixture then part of the milk, beginning and ending with the flour mixture, making sure it is well combine after each addition.
Evenly distribute the batter between 12 muffin tins, filling approximately ¾ of the way full.
Bake for 15-17 minutes. The tops will be golden brown and a cake tester will come out clean. Allow to cool for a few minutes.
In the meantime, combine the cinnamon and sugar. Using a pastry brush, brush melted butter on top of each muffin followed by a sprinkling of the cinnamon and sugar. Alternatively, you can brush the top with butter and then sift powdered sugar over the top.
Best served the day of baking and slightly warm.