plum blueberry upside-down cake

I don't know why I didn't think my kids would care much for this cake but the whole family really enjoyed it and I thought this cake was pretty as a picture.  I was happy to get to use my poppy seeds for this recipe because I bought a jar that cost me a pretty penny a couple of weeks ago so I want to use them!  This is perfect for a brunch or you might want to add a scoop of vanilla ice cream on top and serve as a dessert.

plum blueberry upside-down cake (Everyday Food Magazine, September 2011)
10 tablespoons unsalted butter, room temperature, plus more for pan
1 cup packed light brown sugar, divided
1 large firm but ripe black plum (I used 2)
1 1/2 cups blueberries
1 large egg plus 1 large egg white
1 tablespoon finely grated orange zest
1 1/2 cups all-purpose flour (spooned and leveled)
1 teaspoon poppy seeds
3/4 teaspoon baking powder
1/4 teaspoon baking soda
1/2 teaspoon fine slat
1/2 cup buttermilk (I put a little vinegar in regular milk)
1 teaspoon pure vanilla extract

Preheat oven to 375.  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 (make sure the fruit is packed snugly in pan).
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 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 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.