1. Preheat the oven to 350 degrees F (175 degrees C). Grease an 10-inch springform pan.

2. Sprinkle prepared plums with lemon juice in a bowl.

3. Combine flour, butter, sugar, eggs, buttermilk, baking powder, salt, and lemon zest in the bowl of a food processor; pulse until blended. Process on low speed until smooth. Pour batter into the prepared springform pan.

4. Arrange plums, cut-side down, on the batter. They can touch or even overlap; the more plums you can fit, the better the cake. Dust with cinnamon.

5. Bake in the preheated oven until a toothpick inserted into the center comes out clean, 45 to 50 minutes. Cool on a wire rack for 15 to 20 minutes before serving. Run a table knife around the edges to loosen. Invert carefully onto a serving plate. Dust with confectioners' sugar.

Cook's Notes:

I usually soften butter in the microwave for 12 to 15 seconds (exact time will depend on your microwave), if I forget to take the butter out of the refrigerator in advance.

If you are making the cake by hand, cream softened butter and sugar together, add eggs one at a time, and then the lemon zest. Add the flour (sifted together with baking powder and salt) alternately with the buttermilk, adding a little at a time. Mix until light and smooth.