Preheat your oven to 400°F and line 6 muffin cups with paper liners. While the oven heats, make the streusel by melting 3 tablespoons of butter and combining it with 2/3 cup flour, 4 tablespoons brown sugar, 1/4 teaspoon salt, 3/4 teaspoon cinnamon, and 1/8 teaspoon nutmeg until the mixture resembles coarse crumbs. Transfer the streusel to the refrigerator so it stays cold and crumbly while you prepare the batter.

In a medium bowl, whisk together 1 1/4 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon salt, and 3/4 teaspoon cinnamon. Set this dry mixture aside—this will be folded into the wet ingredients later.

Melt 4 tablespoons of butter and cool it to approximately 90°F (you want it warm but not hot to the touch). In a separate bowl, whisk the room-temperature egg with 7 tablespoons sugar for about 30 seconds until combined. Pour in the cooled melted butter, 6 tablespoons sour cream, 2 tablespoons milk, and 1 teaspoon vanilla, stirring gently to combine. Let this mixture rest for 5 minutes—this resting period allows the ingredients to fully incorporate and helps develop a tender crumb. I like to use Kerrygold butter for its rich flavor, which really enhances the final muffin.

Gently fold the dry ingredient mixture from Step 2 into the wet mixture from Step 3 using a spatula or wooden spoon, stirring just until combined—overmixing will make tough muffins. Let the batter rest for 10 minutes; this allows the flour to fully hydrate and creates a more tender texture.

While the batter rests, cut 5 ounces of fresh rhubarb into 1/2-inch pieces. Toss the rhubarb pieces lightly in a tablespoon of flour (from your pantry, not listed separately) to prevent them from sinking and to reduce excess moisture. After the 10-minute rest, gently fold the floured rhubarb into the batter. Divide the batter evenly among the 6 prepared muffin cups, filling each about three-quarters full. Top each muffin with a few pieces of the remaining rhubarb, then sprinkle generously with the cold streusel mixture from Step 1.

Bake the muffins at 400°F for 5 minutes to set the structure quickly, then reduce the oven temperature to 350°F and continue baking for 12–15 minutes more, until a toothpick inserted into the center comes out clean or with just a few moist crumbs. The initial hot bake creates a nice dome, while the lower temperature prevents the tops from browning too quickly. I find this two-stage baking method gives the most evenly baked muffins with a tender interior.

Remove the muffin pan from the oven and let the muffins cool in the pan for 5 minutes. This brief cooling period allows them to set slightly so they release cleanly from the liners. Transfer the muffins to a wire rack to cool completely before serving.