Make the topping: In a food processor combine the flour, the brown sugar, the butter, and the cinnamon and pulse the motor until the topping is combined well and crumbly.
Preheat the oven to 350 degrees and butter and flour an 8-inch round or square baking pan. In a bowl with an electric mixer cream together the butter and the granulated sugar until the mixture is light and fluffy, add the eggs, 1 at a time, beating well after each addition, and sift in the flour, the baking powder, and the salt. Beat the batter until it is just combined and spread it evenly in the pan. Arrange the peach slices in slightly overlapping rows over the batter, sprinkle the topping over them, and bake the cake in the middle of the oven for 50 minutes to 1 hour, or until a tester comes out clean. Serve the cake warm or at room temperature.