Preheat oven to 350°F (175°C). Grease a 9-inch round cake pan or deep pie dish.
In a small saucepan, melt butter and stir in brown sugar until smooth. Pour into the prepared pan.
Evenly spread chopped rhubarb over the brown sugar mixture.
In a mixing bowl, cream softened butter and sugar until light and fluffy.
Add egg and vanilla, beat well.
In a separate bowl, whisk flour, baking powder, and salt.
Gradually add dry ingredients to the butter mixture, alternating with milk. Mix until just combined.
Spoon batter gently over the rhubarb layer and smooth the top.
Bake for 40–45 minutes or until a toothpick inserted in the center comes out clean.
Let cool in pan for 10 minutes. Invert onto a plate while still warm.
