Make the streusel by melting the butter and stirring in the flour, brown sugar, and salt until crumbly. Set aside.
Preheat oven to 400 F and line a muffin tin with 12 muffin liners. Stir together the 3 tablespoons brown sugar and diced rhubarb in a small bowl, set aside. Whisk together the flour, sugar, baking powder, cinnamon, salt in a large bowl and set aside. Beat together the butter, sour cream, milk, eggs and vanilla extract.
Pour the wet ingredients into the dry ingredients and gently fold together. Do not over mix, there should still be streaks of flour. Then gently stir in the rhubarb. Divide the batter between the muffin tins, (sometimes it makes enough for 13-14 muffins) and then top with the streusel. Bake at 400 for 5 minutes. Then, without opening the oven, turn the heat down to 350 and bake for 12-15 more minutes or until a toothpick inserted comes out clean.