3 tablespoons (38 grams) light or dark brown sugar
1/4 teaspoon ground cinnamon
Pinch of nutmeg
Pinch of salt
3 tablespoons (42 grams) unsalted butter, melted
Muffin
1 large egg
1/4 cup (50 grams) light or dark brown sugar
3 tablespoons (38 grams) granulated sugar
5 tablespoons (71 grams) unsalted butter, melted and cooled to lukewarm
3/4 cup (177 ml) sour cream
1 cup (approx. 120 grams) white whole wheat flour or whole wheat pastry flour (see Note)
1/2 cup (63 grams) all-purpose flour
1 1/2 teaspoon baking powder
1/4 teaspoon baking soda
1/4 teaspoon salt
1 cup diced rhubarb, in 1/2-inch pieces (from about 6 to 8 ounces of stalks)
Directions
Preheat oven to 375°F. Butter 12 muffin cups.
Make streusel: In a small dish, stir together flours, sugars, spices and salt. Stir in butter until crumbly. Set aside.
Make muffins: Whisk egg in the bottom of a large bowl with both sugars. Whisk in butter, then sour cream. In a separate bowl, mix together flours, baking powder and baking soda and stir them into the sour cream mixture, mixing until just combined and still a bit lumpy. Fold in rhubarb and 1/3 (feel free to eyeball this) of the streusel mixture.
Divide batter among prepared muffin cups. Sprinkle each muffin with remaining streusel, then use a spoon to gently press the crumbs into the batter so that they adhere. Bake for 15 to 20 minutes, until tops are golden and a tester inserted into the center of muffins comes out clean. Rest muffins in pan on cooling rack for two minutes, then remove muffins from tin to cool them completely.