This moist and delicious cake turns out of the pan with a sweet, jamlike layer of fruit on top.
Preheat oven to 375 degrees. Lightly butter an 8-inch round cake pan (2 inches deep); line bottom with parchment paper. Melt 2 tablespoons butter and pour into pan; swirl to coat paper. Sprinkle 1/2 cup brown sugar evenly over butter. Arrange plum evenly in pan, then add blueberries, filling in any gaps.
In a large bowl, using an electric mixer, beat 8 tablespoons butter and 1/2 cup brown sugar on high until light and fluffy, about 3 minutes. Beat in egg, egg white, and orange zest until combined. In a medium bowl, whisk together flour, poppy seeds, baking powder, baking soda, and salt. With mixer on low, beat flour mixture into butter mixture until combined. Beat in buttermilk and vanilla until smooth.
Transfer batter to pan. Tap pan firmly on counter several times to remove air bubbles; smooth top.
Bake until cake is deep golden brown, juices are bubbling around edge, and a toothpick inserted in center comes out clean, about 45 minutes, rotating pan halfway through. Let cake cool in pan on a wire rack, 1 hour. Run a knife around edge of pan and invert cake onto a serving plate. Serve warm or at room temperature. (Store, covered, at room temperature, up to 2 days.)
Make sure the fruit is packed snugly in the pan; it will shrink and lose volume as it bakes.