Preheat oven to 350°F.
Toss rhubarb & flour in a small bowl.
Pour ¼ cup melted butter into a 9" round pan. Top with brown sugar. Arrange the rhubarb mixture on the brown sugar. Set aside.
Combine sugar & melted butter. Stir in egg & lemon juice.
In a small bowl, combine flour, baking powder, lemon zest, & a pinch of salt.
Add flour mixture alternately with the milk to the butter mixture. Stir just until combined.
Spread the batter over the rhubarb mixture and bake 32-36 minutes or just until a toothpick comes out clean.
Remove from the oven, let cool in the pan 15-20 minutes. Run a knife along the edge of the cake and invert onto a platter. Serve warm with ice cream if desired.
