Preheat the oven to 350 and prepare a 9-inch springform pan with a layer of parchment paper.
Using a microwave-safe measuring cup, heat the sugar and the water for 1-2 minutes, until the sugar is dissolved. Using a pastry brush, brush a layer of the sugar syrup mixture on top of the parchment. Set to the side.
Slice the oranges in rounds to fit in the bottom of the pan. It is easier to slice the citrus rounds and then cut off the peel and the pith using a pairing knife. When the citrus rounds are ready, arrange them in the bottom of the pan, leaving no gaps. Brush the remaining sugar syrup on top of the citrus. Set the pan to the side.
Make the cake. In a medium-sized bowl, combine the flour, baking powder, baking soda, and salt. Set to the side.
In a large bowl with a hand mixer or in a stand mixer with a paddle attachment, cream the butter with the sugar, about 2-3 minutes or until light and fluffy. Add the eggs, zest, orange juice, and vanilla, scraping down the sides of the bowl as needed.
Slowly add the dry ingredients to the butter mixture, scraping down the bowl as needed. Add in the sour cream, just mixing to combine.
Pour the cake batter carefully over the citrus rounds and place the cake in the oven to bake for 55-65 minutes. The cake will be done when the edges start to pull away from the pan, and the cake is golden brown.
Let the cake cool in the pan completely. When the cake is fully cool, turn the cake over on a plate or cake plate to see the finished product.
