Preheat the oven to 350 degrees Fahrenheit and prepare two muffin tins with cooking spray or paper liners.

Prepare the dry ingredients: In a large mixing bowl, whisk together the cake mix and the flour. Then, remove 1 cup of the mixture and place it to the side. You will use it later to make the streusel.

Add the baking powder and cinnamon, whisk to combine.

Prepare the wet ingredients: In a separate medium bowl, mix together the wet ingredients: eggs, ⅓ cup of the melted butter, and the half and half.

Combine: Then, add the wet ingredients to the dry ingredients. Beat together for about one minute, occasionally using a rubber spatula to scrape the sides and bottoms of the bowl. Lumps are okay.

Pour the batter into the prepared muffin cup, filling each cavity about ½ full with batter.

Make the streusel: Combine brown sugar, cinnamon and remaining cake mix in a small bowl. 

Use a fork to mix in the rest of the melted butter until a thick, sticky mixture forms. Whisk to combine until it forms into coarse crumbs. 

Crumble the cinnamon sugar mixture evenly over the top of the batter.

Bake for 15-18 minutes, or until the edges begin to turn golden brown and a toothpick inserted into the center of the muffin comes out clean.

Cool for 5 minutes before removing from the muffin tin. Serve warm, or place on a wire rack to cool completely.