• Heat oven to 425ºF (218ºC) with rack set in middle position.
• Line a standard 12-cup muffin tin with cupcake liners then coat liners with nonstick spray.

Streusel

• In a medium bowl, combine ½ cup (63g) flour, ¼ cup (50g) brown sugar, 2 tablespoons (25g) granulated sugar, ⅛ teaspoon nutmeg, and pinch salt. Using your fingers, work butter into dry ingredients by pinching and smearing until a stiff mixture comes together. Press into a single mass then transfer bowl to refrigerator.

Muffin Batter

• In a large bowl, whisk together 2½ cups (312g) flour, 1 tablespoon (12g) baking powder, ½ teaspoon nutmeg, ¼ teaspoon cardamom, and ¼ teaspoon salt. Add 2 cups (230g) rhubarb and toss to combine; set aside.
• In a separate mixing bowl, whisk together ¾ cup (180g) buttermilk, ¾ cup (150g) granulated sugar, ½ cup (100g) brown sugar, ½ cup (110g) oil, 2 eggs, and 2 teaspoons lemon zest until smooth.
• Add wet mixture to dry and flour mixture and mix with a rubber spatula just until combined (a few pockets of dry flour is OK).
• Using a measuring cup or large scoop, divide batter evenly between muffin liners (cups will be full). Top each with a few pieces of rhubarb.
• Break streusel into pieces (¼-inch to ½-inch in size) and divide between each well of batter, lightly pressing into batter to secure.
• Bake muffins for 5 minutes at 425°F (218°C) then, keeping muffins in oven, reduce oven temperature to 350°F (177°C) and bake until muffins are golden around the edges, tops are set, and a toothpick inserted in the centers comes out clean, 21–23 minutes more. Total bake time is about 26-28 minutes.
• Let cool in pan 10 minutes, then transfer muffins to a wire rack to cool completely.