Streusel:
Melt butter and stir in four, brown sugar and salt until crumbly. Set aside
Muffins:
Preheat oven to 400 and line a muffin tin with 12 liners.
Sir together 3 T brown sugar and diced rhubarb. Set aside
Whisk together flour, sugar, baking powder, cinnamon and salt in a large bowl. Set aside
Beat together butter, sour cream, milk, eggs and vanilla
Pour wet ingredients into dry ingredients and fold together. Do not over mix. There should still be streaks of flour in the batter.
Gently stir in the rhubarb
Divide batter into muffin tins
Top with streusel topping
Bake at 400 for 5 minutes, then, without opening the oven reduce heat to 350 and bake another 15 to 20 min. Until a toothpick comes out clean.