Preheat the oven to 350ºF. Grease a 10-inch cake or tart pan generously with butter. Sprinkle it with 1 Tablespoon of the sugar. Set aside.
Beat the almond paste and ½ cup of the sugar in the bowl of a mixer fitted with the paddle.
Add
the butter and beat on medium high speed until the butter is well
incorporated into the paste mixture, about 2 to 3 minutes.
Beat in the eggs, flour, baking powder and salt.
Scrape the batter into the prepared pan, leveling it with a knife or spatula.
Arrange the plums, skin side up in a circular fashion covering the entire surface of the batter. Place smaller slices in the center. Sprinkle with the cinnamon sugar mixture.
Bake
until the batter puffs and browns and a toothpick inserted into the
center of the cake comes out clean, about 40 to 50 minutes.
Serve the cake while still slightly warm cut into wedges with whipped cream or ice cream. Drizzle some of the Fresh Plum Sauce over the tart if desired.
To make the Fresh Plum Sauce:
Heat
the sliced plums, sugar and lemon juice in a shallow skillet over
medium low heat until the plums loose some juice and the sugar
dissolves, about 5 to 6 minutes. Serve warm or at room temperature.