Place the unsalted butter, sugar, flour, and marzipan into a bowl.
Use your fingers or a fork to rub the mixture together.Set aside.
Preheat oven to 350ºF.
Grease a 12 cup muffin tin.
Whisk together the flour, baking powder, and salt. Set aside.
In a separate bowl, whisk together the milk, oil, egg, sugar, and vanilla.
Mix in the dry ingredients until the batter comes together. Do not over mix.
Fold in the rhubarb.
Divide the batter evenly among the 12 cups in a muffin tin.
Sprinkle the streusel on top.
Bake for 20 minutes or until a knife inserted comes out clean.
Transfer to a wire rack and let cool. Lovely if served slightly warm.