Step 1Cut the plums in half and remove the pits. Cut the halves into quarters or, with larger plums, into fat wedges. Heat the oven to 425 degrees. Butter a large, shallow baking dish (I use a round gratin dish, about 14 inches across), sprinkle it with 2 tablespoons of sugar and arrange the plum wedges in one layer, fitting them close together. Put them in the oven for 10 minutes.
Step 2Meanwhile, sift together the flour, baking powder and salt. Beat the eggs with 3/4 cup of sugar until creamy, then beat in the flour mixture, the warmed milk, melted butter, vanilla and lemon zest to make a smooth batter.
Step 3Pour the batter over the plums, slowly and evenly. Combine the remaining 2 tablespoons of sugar and the cinnamon and sprinkle randomly over the top of the batter. Return the dish to the oven and reduce the heat to 400 degrees.
Step 4Bake the clafouti until it is puffed and golden, about 35 minutes, but check it after 15 minutes. This clafouti has a little baking powder in the batter, so don't be surprised to see it rising in high, billowy shapes. If it bubbles up too much, it can be pierced with a sharp knife point to release some air. If it is browning too quickly, lay a sheet of foil loosely over the top for some of the time. The clafouti will sink gradually when you take it out of the oven.