Generously butter the bottom and sides of a 9-inch square baking pan, or line with parchment paper if you plan to lift the entire cake out of the pan for serving presentation. I use two sheets of parchment paper, placed crosswise in the pan with plenty hanging over the top to easily lift out the cake.
Place an oven rack in the center position and preheat the oven to 350° F.
In a medium mixing bowl, whisk together the flour, 1/ 4 cup granulated sugar, baking powder and salt. In a small bowl combine the first beaten egg, milk, and melted butter. (I use the one-cup liquid measuring cup for this. Measure the milk, add the beaten egg and melted butter, then stir.)
Add the wet mixture to the flour mixture and mix until just combined with no streaks of flour showing. But don’t over mix. The batter will be thick.
Spread this batter evenly into the prepared 9-inch pan and bake for 15 minutes.
Meanwhile prepare the topping by adding the remaining beaten egg, 1/ 4 – 1/ 3 cup of granulated sugar, and sour cream together in a small bowl. Mix well. Slice the peaches and set aside.
After the initial 15 minutes, of baking remove the partially baked cake from the oven and set on a heatproof surface or hot pad. Spread the sour cream mixture evenly over the cake then arrange the peaches in a single layer, decorative pattern. Sprinkle with cinnamon.
Return to the 350° F oven and continue baking for another 15minutes. Cake is done when a toothpick inserted in the center of the cake comes out clean. Do not over bake.
Serve warm or chilled.
Notes
Store any leftover cake covered in the refrigerator for up to 2 days. The cake will absorb the sour cream topping and become more moist over time. This isn't necessarily a bad thing, think tres leches cake!