Preheat oven to 350°F (175°C). Grease and flour a 9-inch baking pan.
In a large bowl, cream together the softened butter and 1.5 cups of granulated sugar until light and fluffy.
0.5 cup unsalted butter
Beat in the eggs one at a time, then stir in the vanilla extract.
0.5 cup unsalted butter
In a separate bowl, whisk together the flour, baking powder, and salt.
0.5 cup unsalted butter
Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined.
0.5 cup unsalted butter
In a small bowl, toss the chopped rhubarb with 0.25 cup of granulated sugar and cinnamon.
0.5 cup unsalted butter
Spread the rhubarb mixture evenly over the cake batter in the prepared pan.
Bake for 40-50 minutes, or until a wooden skewer inserted into the center comes out clean.
Let cool in the pan for 10 minutes before inverting onto a wire rack to cool completely.
Notes
Serve warm with a dusting of powdered sugar or a dollop of whipped cream.