Preheat your oven to 425°F and line a muffin tin with paper liners or grease it well—I like to use liners for easy cleanup. Chop the rhubarb into 1/2-inch pieces and set aside. Zest the lemon and juice it, measuring out what you need. Bring your eggs to room temperature if they aren't already, as this helps them incorporate smoothly into the batter and creates a more tender crumb.

In a medium bowl, whisk together the flours (1 1/4 cups white flour plus 3/4 cup flour), both sugars (3/4 cup white sugar and 1/2 cup brown sugar), baking soda, baking powder, and salt. Whisking the dry ingredients together distributes the leavening agents evenly, which ensures your muffins rise uniformly and have a consistent crumb structure. Set this mixture aside.

In a separate bowl, combine the cooled melted butter with the lemon zest and let it sit for 30 seconds to bloom the zest flavors. Add the room-temperature eggs and whisk until combined, then stir in the Greek yogurt, vanilla, milk, and lemon juice. The yogurt adds tanginess and moisture while keeping the muffins tender—I prefer Greek yogurt because its thickness prevents the batter from becoming too wet. Mix until just combined but don't overmix.

In a small bowl, combine 1/3 cup flour, 3 tbsp brown sugar, and 1/4 tsp ground cinnamon. Add the cold butter cubes and use a fork to work the mixture together until it resembles coarse breadcrumbs with pea-sized pieces of butter throughout. The cold butter creates little pockets that brown and crisp in the oven, giving the topping texture and flavor. Set the streusel aside until ready to use.

Pour the wet mixture from Step 3 into the dry ingredient mixture from Step 2 and fold together gently with a spatula until just combined—don't overmix, as this keeps the muffins tender. Fold in the chopped rhubarb until evenly distributed. Divide the batter evenly among the muffin cups, filling each about two-thirds full, then top each with a generous spoonful of the streusel topping from Step 4, pressing it gently onto the batter.

Bake at 425°F for 5 minutes to set the structure and help the muffins rise quickly, then lower the oven temperature to 350°F and bake for an additional 18 minutes until a toothpick inserted into the center comes out with just a few moist crumbs and the tops are golden brown. The high initial heat creates lift and helps the streusel brown, while the lower finishing temperature ensures the centers cook through without the tops burning. Let the muffins cool in the tin for 5 minutes before turning them out onto a wire rack.