Streusel
1/4 cup (31 grams) all-purpose flour
1/4 cup (28 grams) white whole wheat flour
1 tablespoon (13 grams) granulated sugar
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)