1/2cupunsalted butter (1 stick) at room temperature plus extra for the cake pan
1 1/3cupunbleached all-purpose flourplus extra for the cake pan
1/2cupfinely ground cornmeal
1 1/2teaspoonsbaking powder
1 1/2teaspoonskosher salt
1/3cuppacked light brown sugar(about 2 ounces)
3largeeggsat room temperature
1/3cupfreshly-squeezed orange juice, whole milk, or almond milkat room temperature
1tablespoonpure vanilla extract
Instructions
Macerate The Fruit: Combine peach slices, rhubarb, lemon juice, and 1/4 cup of the sugar in a large nonreactive bowl, toss to coat, and set aside until juicy, at least 5 minutes.
Prepare The Cake Pan: Heat oven to 350°F and arrange rack in the middle. Coat a 9-inch round cake pan with extra butter and extra flour; tap out excess flour. Then line the pan with a round of parchment paper, cover with additional butter and flour, tap out excess, and set aside.
Make The Polenta Cake Batter: Combine 1 1/3 cups of flour, cornmeal, baking powder, and salt in a large bowl and whisk to combine and break up any lumps; set aside.
Place the 1 stick of butter, brown sugar, and remaining 3/4 granulated sugar in a bowl of a stand mixer fitted with a paddle attachment. Beat on a medium speed, until the butter is creamy and consistency of wet sand. Add the eggs, one at a time, letting each egg fully incorporate before adding the next. Add the juice or milk, the vanilla extract, and mix until combined (it may look curdled at this point but don't worry about it), about 1 minute. Reduce mixer to low and add dry ingredients, little by little, and mix until just incorporated.s
Assemble The Polenta Cake: Arrange the fruit mixture on the bottom of the cake pan (including any excess juices) then spoon the batter evenly over the fruit and spread it out evenly. Bake until the cake is golden brown, evenly risen, springs back when lightly pressed, and a cake tester or skewer inserted in the middle comes out with only a few crumbs, about 35 to 45 minutes.
Cool in pan on a wire rack 15 minutes. Place a plate upside down on top of cake pan; invert cake onto plate. Serve warm or at room temperature, preferably with ice cream or whipped cream.