Preheat oven to 350ºF (177ºC) with a rack in the center.
Place the butter in a 2- or 2 ½-quart baking dish (any shape), or a 9- or 10-inch cast-iron skillet or ceramic pie plate. Transfer the baking dish to the oven and let the butter melt, about 5 minutes. Remove from the oven and carefully tilt the pan around to make sure the dish and sides of the baking dish are coated with melted butter. Pour the excess butter into a small bowl (don't scrape it out of the pan). Leave it aside.
In a large bowl, mix the sourdough starter, eggs, cream, 50 grams (¼ cup) sugar, salt, almond extract and cinnamon if using until very smooth. Slowly pour in the cooled melted butter.
Spread the fruit in the baking dish, then pour over the batter. Sprinkle with remaining 13 grams (1 tablespoon) sugar.
Bake until the clafoutis is puffed, set, and beginning to turn golden around the edges, 25 to 30 minutes (the wider the baking dish, the shorter the cooking time.). Let cool for at least 15 minutes (it will deflate a bit and may crack, don't worry!) or up to 2 hours before dusting with powdered sugar if desired. Use a large spoon to remove pieces, or cut into pieces. Leftovers are stored covered in the refrigerator for up to 2 days.
