Set a rack in the middle of the oven and preheat the oven to 350 °F / 177℃. Grease a 9-inch baking pan or cast-iron pan with butter.
Place 2 cups pitted and thinly sliced peaches in a medium-sized bowl. Toss with 2 teaspoons vanilla paste, 1 tablespoon sugar, and the zest from 1 lemon. Transfer to the baking pan and arrange evenly.
To the base of a blender, add remaining ⅓ cup of granulated white sugar, 3 large eggs, 1 ¼ cups whole milk, 1 tablespoon unsalted butter(melted), ¼ teaspoon kosher salt, and ½ teaspoon almond extract. Blend until combined.Add ⅔ cup all-purpose flour and blend until smooth.
Slowly and evenly pour the clafoutis batter over the peaches. Top with 1 tablespoon demerara sugar, if using.Note: If you see any large air bubbles, you can poke them with a fork or a chopstick.
Bake the clafoutis for 40 minutes to an hour until the edges are golden-brown and the middle is just set, but still slightly wobbly. A toothpick placed into the center of the pan should be clean (or with a few moist crumbs attached).Note: I recommend checking on the clafoutis after 30 minutes, especially if using a shallower (10-inch pan). In my 9-inch casserole dish, it took me closer to an hour.
Remove the clafoutis from the oven and let cool for 10 - 15 minutes until still warm, but not hot.
Dust with powdered sugar, cut into slices, and serve warm.
Notes
Recipe adapted from Julia Child's Mastering the Art of French Cooking. This recipe was developed using metric measurements, so for best results, use a scale.Ingredient Notes: I'm a big fan of using vanilla paste in desserts, as I think it adds a more intense aroma and flavor. If you don't have this, you can substitute an equal amount of extract.Equipment Notes: A blender is the easiest way to prepare this dessert, but you can also use a large mixing bowl and a whisk. If using a whisk, make sure to slowly sift the flour into the batter, and gently whisk until no clumps remain.Storage: Leftover clafoutis can last 3-4 days in the fridge; slice pieces in an airtight container.Make-Ahead Instructions: You can also bake the dessert up to 1 day in advance. Reheat before serving until warmed through.