Heat your oven to 400°F and prepare a muffin tin by greasing it or inserting liners. While the oven preheats, slice the rhubarb into 1/2-inch pieces, rinse the blueberries, and set both aside separately. Measure out all your dry ingredients (flour, baking powder, baking soda, and salt) into one bowl and set aside—this mise en place approach ensures you won't miss anything once you start mixing.

Melt the 1/2 cup butter and let it cool to room temperature (about 2-3 minutes). In a large bowl, whisk together the cooled melted butter, sugar, room-temperature yogurt, and eggs until well combined and slightly fluffy, about 1-2 minutes. I like to use high-quality butter like Kerrygold because it gives the muffins a richer, more tender crumb—it really makes a difference.

Add the dry ingredient mixture from Step 1 to the wet ingredients from Step 2, folding gently with a spatula until just combined. Don't overmix—a few small lumps are okay and will result in more tender muffins. Stop mixing as soon as you don't see streaks of flour.

Gently fold the sliced rhubarb and blueberries from Step 1 into the batter, being careful not to crush the berries. Divide the batter evenly among the prepared muffin cups, filling each about 3/4 full. Reserve a few pieces of rhubarb and some blueberries to place on top of each muffin before the topping.

While the muffin cups are being filled, mix together the 5 tablespoons melted and cooled butter, brown sugar, 3/4 cup flour, and cinnamon in a small bowl until the mixture resembles coarse breadcrumbs. Sprinkle this topping evenly over each muffin, then gently press a few reserved rhubarb pieces and blueberries into the top of each muffin for a beautiful finish.

Place the muffin tin in the preheated 400°F oven and bake for 14-18 minutes, until the tops are golden brown and a toothpick inserted into the center of a muffin comes out clean or with just a few moist crumbs. I like to rotate the pan halfway through baking to ensure even browning, especially if your oven has hot spots. Remove from the oven and let the muffins cool in the tin for 3-5 minutes before serving.