Preheat the oven to 350 degrees. Grease a 9x5 loaf pan.
In the bowl of a stand mixer, cream together the butter and sugar until light and fluffy. Beat in the egg and vanilla until well combined.
Combine the flour, baking powder, and salt in a small bowl.
Add the flour mixture a little at a time to the butter mixture, alternating with the buttermilk.
Fold in the cranberries by hand.
To make the streusel, mix together the flour, brown sugar, sugar, and cinnamon until combined.
Stir in the melted butter until well combined and crumbly.
Sprinkle the streusel over the loaf of bread and press down lightly to adhere the crumbs.
Bake for 50-60 minutes or until a butter knife inserted in the center comes out mostly clean.
Cool completely before cutting.