Preheat the oven to 350 degrees F. Butter two 9-inch cake pans and sprinkle with extra sugar. Shake the pans around for good coverage, then dump out the excess sugar. Set aside.
Using an electric mixer, cream 1 cup (2 sticks) of softened butter with 2 cups of granulated sugar. Beat until light and fluffy. Then add the eggs and vanilla. Scrape the bowl and beat until smooth.
Mix the flour, baking powder and salt in a separate bowl. Once mixed, turn the mixer on low and slowly add the flour mixture along with the buttermilk. Zest one orange and beat the zest into the batter.
Melt the last stick of butter and pour evenly into the two cake pans. Then evenly divide the brown sugar and sprinkle to coat the bottom of each pan.
Cut the tops and bottoms off the oranges. Gently shave the skin off the oranges, then slice the naked oranges into 1/4-inch rounds. Arrange the orange slices over the bottom of one cake pan. Trim pieces if needed to make them fit over the entire surface. Then pour the blueberries into the other cake pan and shake to level.
Pour the batter into both cake pans. Because the blueberries will come up a little higher than the oranges, it's ok to add a little more batter to that pan. Bake for 30-40 minutes. Check by inserting a toothpick into the center of both cakes. When it comes out clean, the cakes are done. The blueberry cake may take a few minutes longer than the orange cake.
To assemble: Allow both cakes to cool in the pan for at least 20 minutes. Both cakes need to be close to room temperature so they don't fall apart! Flip the orange cake out onto a cake plate. Wiggle the pan to release the oranges. If a slice or two stick to the pan, piece it back in it's proper place. Then carefully flip the blueberry cake layer on top of the orange cake layer. Wiggle to release. Serve with whipped cream or vanilla ice cream!