• Preheat oven to 350F.
• Pour the butter into a 9-inch springform cake pan. Use your finger to run a bit of butter around the side of the pan so it's well-greased.
• Evenly sprinkle the brown sugar over the butter.
• Arrange the orange slices in the pan to cover the bottom.

To Make the Cake

• Whisk flour, granulated sugar, baking powder, baking soda, and salt in a large bowl. Add eggs, egg yolk, buttermilk, butter, and blood orange zest and juice; mix until no dry spots remain (a few lumps are okay; do not overmix).
• Pour batter over oranges in pan and bake until top is golden brown and cake pulls away from sides of pan, 35–40 minutes.
• Place pan on a wire rack and allow cake to cool for at least 10 minutes before inverting, slicing, and serving.
• To Make the Blood Orange Slices for Garnish
• In a medium skillet, combine the water and sugar and bring to a boil. Add the orange slices and cook over moderate heat, turning them occasionally, until the liquid is reduced to a thin syrup and the orange slices are translucent, about 20 minutes.
• Reduce the heat to moderately low and simmer until the syrup is thick and the slices are tender but still intact, turning occasionally, about 10 minutes.
• Transfer the orange slices to a rack to cool. Reserve the syrup for another use.
• Once cooled; top the cake with the additional orange slices.
• Serve with a dollop of orange zest whipped cream if desired.
• Make Ahead - The candied orange slices can be refrigerated for up to 2 weeks.