Preheat your oven to 375-degrees. Grease or line your muffin tins (this will make 12-18 muffins depending on the size of your tins). Cream the butter and sugar in a small bowl. In a larger bowl, beat the egg, orange zest and vanilla. Add in the butter-sugar mixture and stir to combine. Then add in the dry ingredients, and the milk and mix until well combined. Fold in the sweet potato puree (and the walnuts if you're using them). Scoop the muffin batter into the prepared tins - I like to use an ice cream scoop. To make the streusel topping, pulse the softened butter with the flour, brown sugar and cinnamon until it is a bit crumbly. If you're a walnut fan, the tiny chopped ones area pretty tasty in this streusel topping, too. Alas, I'm the only nut lover in my house... Sprinkle the topping over the muffins, gently pressing down so that you don't loose all that yummy goodness when you unwrap your muffin. Bake for 20-25 minutes or until a tester (toothpick) comes out clean. Cool in the pans for 5 minutes, then remove to a rack to cool. Now, if you want to make this really special, drizzle on a little Orange Glaze (1/2 cup powdered sugar, 1-1/2 tablespoons orange juice, zest of 1 small orange). - See more at: http://www.jugglingactmama.com/2013/02/sweet-potato-muffins-with-streusel.html#sthash.y1wxQbOx.dpuf