Preheat oven to 400 degrees Farenheit.

Line a muffin tin with twelve paper or silicone liners and set aside.

Brown the butter: add unsalted butter to a small skillet (stainless steel works well) and place on the stove over medium-low heat. Allow to melt and cook; the butter will pop and froth. When the popping stops, stir gently with a spatula and watch for browning. Shortly after the popping stops, brown bits will begin to appear and the bottom. When a lot of brown bits begin to appear and butter darkens and is very aromatic, remove from heat and allow to cool

In a large mixing bowl, whisk together flour, sugars, salt, baking powder, and cinnamon until combined.

In a bowl or large liquid measuring cup, beat together buttermilk, butter, eggs and vanilla until combined.

Pour the wet ingredients into the dry ingredient mixture. Combine, but do not overmix! Stop when a couple small flour streaks remain.

Add rhubarb and mix.

Scoop batter into prepared muffin tin.

Make the streusel topping: in a medium mixing bowl, whisk together flour, sugar and cinnamon.

Cut cold butter into small cubes and add to dry ingredients. Use a fork or a pastry cutter to cut butter into flour mixture until mixture forms large coarse crumbs.

Sprinkle streusel over muffin batter.

Place pan in oven, bake for 16-19 minutes. Muffins will be golden brown. Remove pan to a cooling rack.

Enjoy warm or room temperature. Muffins are best fresh, but they can be frozen for later!