Plum-Almond Upside-Down Cake

To make topping: Preheat oven to 350°F.

Combine 3/4 cup sugar and water in a heavy-bottomed saucepan. Bring to a simmer over low heat, swirling pan occasionally, until sugar dissolves. Increase heat to medium, cover and cook for 1 minute. Uncover and cook, without stirring, until syrup turns a deep amber color, 8 to 10 minutes. (Gently swirl pan if syrup is coloring unevenly). Immediately pour caramel into a 9-inch round cake pan. Swirl pan to coat bottom evenly with caramel. (Use caution, as caramel will be very hot). Cool until set, about 10 minutes.

Quarter plums lengthwise, removing pits. Arrange, cut-side down, in concentric circles over caramel; set aside.

To make cake: Whisk flour, baking powder, baking soda and salt in a bowl.

Combine buttermilk, almond paste, oil and vanilla and almond extracts in a food processor. Process until smooth; set aside.

Combine egg, egg white and 2/3 cup sugar in a large mixing bowl; beat with an electric mixer on high speed until thick and pale, 5 to 10 minutes. Alternately fold dry ingredients and buttermilk mixture into egg mixture with a whisk, making 3 additions of dry and 2 of liquid. Pour batter over plums.

Bake cake until golden and a skewer inserted in the center comes out clean, 35 to 40 minutes. Let cool in the pan on a wire rack for 10 minutes. Loosen edges and invert onto a serving plate. Let cool to room temperature.

Shortly before serving, heat jelly in a small saucepan over low heat, stirring until melted. With a pastry brush, glaze plums with jelly.