In a medium bowl toss together the rhubarb, cornstarch and 1/4 cup granulated sugar.
In a medium sized saucepan, over medium heat, combine the brown sugar and 1/2 stick of butter. Whisk or stir until integrated and bubbling, about 2-4 minutes. Turn off heat and set aside
In a large mixing bowl combine flour, baking powder, sugar, salt, and lemon zest. Cut in the stick of butter with your hands or a pastry blender to the size of small peas.
In a separate small bowl whisk together the yogurt, eggs, lemon juice, and vanilla.
Add the liquids to the flour mixture all at once. Stir until just blended and little or no dry flour bits remain.
Equally divide the brown-sugar mixture into the muffin tins (a little more than a tsp per), then spoon in the rhubarb and its juices (about one heaping tablespoon per). Scoop in the batter so it covers all of the rhubarb. Smooth out the top.
Bake on the middle rack, with a cookie sheet on the rack below to catch any overflow, for 15 to 20 minutes, or until the batter has set and a toothpick inserted into the center of the cake comes out clean. Allow to cool for five-10 minutes. Run a toothpick around the edges of the cakes to makes sure they are loose from the edges. Place a cookie sheet over the muffin tin and then invert the cakes. Thumping the bottoms of the tins if necessary. If you do this too soon the topping will be too hot, wait too long and the cakes might stick.
Individual Rhubarb Upside Down Cakes or Muffins https://www.recipefiction.com/2015/05/individual-rhubarb-upside-down-cakes-or-muffins.html