Preheat the oven to 350 degrees Fahrenheit and spray two muffin tins with nonstick cooking spray or line with cupcake liners
Whisk together the cake mix and flour in a large mixing bowl and then remove 1 cup of the mixture and place it to the side in a small mixing bowl. You will use it later to make the streusel.
In the large mixing bowl, whisk in the baking powder.
Use a hand or stand mixer to beat in the large eggs, ⅓ cup of the melted butter and the heavy cream (or half and half).
Beat together only for about 1 minute or until everything is combined. Use a spatula to scrape the sides and bottom of the bowl if necessary but lumps are ok in this batter.
Pour the muffin mix into the prepared muffin tins, filling each about ½ full with batter.
Whisk the cinnamon into the small mixing bowl with the extra cake mix. Using a fork, mix in the rest of the melted butter until a thick, slightly sticky mixture forms.
Crumble the streusel evenly over the muffins.
Bake for 15-18 minutes or until the edges begin to turn golden brown and a toothpick inserted into the center comes out clean.
Cool for 5 minutes before removing from the muffin tins and serving warm or placing on a cooling rack to cool completely.
Notes
Please refer to my FAQ’s (Frequently Asked Questions) and ingredient list above for other substitutions or for the answers to the most common questions.