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1
Preheat the oven to 350.
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2
Butter a 9-inch-square glass or ceramic baking dish.
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3
In a medium bowl, whisk the flour with the baking powder, cinnamon and salt.
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4
In a large bowl, using an electric mixer, beat the 6 tablespoons of butter with the brown sugar and 1/2 cup of the granulated sugar at medium-high speed until pale and fluffy, about 5 minutes.
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5
Reduce the speed to medium and beat in the vanilla and egg.
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6
Scrape down the side and bottom of the bowl.
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7
At low speed, beat in the dry ingredients and milk in 3 alternating additions, scraping down the bowl as necessary.
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8
In another medium bowl, toss the rhubarb with the remaining 1/2 cup of granulated sugar.
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9
Spread in the bottom of the prepared baking dish.
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10
Dollop the cake batter over the rhubarb and spread it evenly.
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11
Bake for 30 minutes, until the cake is golden brown and a cake tester inserted in the center comes out clean.
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12
Let the cake cool in the pan on a wire rack and serve warm or at room temperature.
