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