Preheat oven to 350 degrees F.
Combine the streusel ingredients in a bowl.
Using a fork, your fingers, or an electric mixer, combine the streusel ingredients. It should look like course sand.
Set the streusel aside.
In a mixing bowl, combine cake mix, egg, sour cream, melted butter, and vanilla.
Combine quick bread ingredients with an electric mixer, scraping sides and bottom of bowl.
Beat until smooth.
Fill four mini loaf pans with the batter and spread to the sides of the pans. Batter will be stiff. (It's helpful to use a large cookie scoop to put the batter into the pans.)
Top each loaf with streusel.
Bake for 40 minutes.
Bread is done when a knife inserted in the middle comes out clean.
Cool, slice, and serve.