rhubarb streusel muffins

INGREDIENTS

• 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
• 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)