Preparation

• Preheat the oven to 400°F. Prepare your muffin pan by adding six paper liners to a 12-cup muffin pan or use a 6-cup muffin pan.
• Make the Brown Sugar Crumble: Melt the butter over low heat. In a small bowl, whisk the flour, brown sugar, and salt together. Pour in the melted butter and mix until clumps form. Refrigerate this mixture until needed.
• Create Muffin Batter: Melt the remaining butter over medium-low heat and allow it to cool until barely warm. In a separate bowl, whisk together the all-purpose flour, baking powder, and salt for about 30 seconds.
• Combine Wet Ingredients: In a large bowl, crack the egg, add the granulated sugar, and whisk until lighter in color. Gradually whisk in the cool melted butter, sour cream, milk, and vanilla extract. Let the batter rest for 5 minutes to enhance the flavors.
• Incorporate Dry Ingredients: Add half of your combined dry ingredients to the wet batter and gently fold until incorporated. Add the remaining dry ingredients and mix on low speed until just combined. Allow the batter to sit for 10 minutes for a fluffier texture.
• Add Rhubarb: Toss sliced rhubarb with a teaspoon of flour to prevent sinking in the batter. Gently fold the rhubarb into the muffin batter.
• Fill Muffin Cups: Divide the batter evenly among the 6 muffin cups. Top each muffin with the remaining rhubarb pieces and a tablespoon of your prepared brown sugar crumble.
• Bake: Place the muffins in the oven and bake at 400°F for 5 minutes. Without opening the door, reduce the temperature to 350°F and bake for another 12-15 minutes until the internal temperature registers 203°F.
• Cool: Let the muffins cool in the pan for 5 minutes. Then, carefully transfer them to a wire cooling rack to cool completely.