Preheat the oven to 350F degrees and adjust the oven rack to the middle position. While the oven is preheating, place the cast iron skillet in the oven to heat up.
In a small mixing bowl or large liquid measuring cup, add the egg, egg yolk, and the other half of the sugar. Whisk vigorously until the eggs are light and pale. Add the vanilla, cream, and milk and continue to whisk until the batter is fully combined.
Sprinkle the flour and salt into the liquid ingredients and mix until just incorporated and smooth. Note - the batter will be 'liquidy', similar to a crepe batter.
Once the oven has preheated, carefully remove the cast iron skillet from the oven. Add the butter to the hot skillet and swirl around to completely coat the bottom and sides of the skillet.
Spoon the juices from the fresh peaches into the hot skillet and swirl around to coat the bottom of the skillet. Then, pour the clafoutis batter into the skillet. Evenly distribute the peach slices into the batter.
Bake the clafoutis for 35 - 40 minutes, or until the edges are golden brown. Note - clafoutis is an eggy dessert and will bake as such. The edges will look crispy and golden while the center will be soft and custard-like.
Allow the clafoutis to cool slightly in the skillet set on a wire cooling rack. Serve warm or at room temperature. Dust the top with powdered sugar if you like, before slicing and serving. The clafoutis is best enjoyed the day it is baked. Enjoy!