Preheat an oven to 350°F. Grease and flour a 9" bundt pan (or spray with non-stick baking spray).
Sprinkle 3/4's of the brown sugar in an even layer into the pan. Make sure to push up some of the sugar to coat the sides. Arrange your sliced citrus in a decorative pattern. You may not use all of the slices, but use as many as you can to create an overlapping layer of fruits. Top with the remaining brown sugar. Bake in the oven for 30 minutes, or until the fruits look glassy and the sugar is molten.
In a medium-size mixing bowl, sift together all the dry ingredients, except the sugar. Set aside.
In a large mixing bowl, cream together the butter, sugar and vanilla. Mix on medium-high for 7 minutes, scraping down the bowl occasionally during mixing.
Add the eggs, one at a time, mixing until fluffy. Scrape down the bowl after each addition is fully mixed.
Add 1/3 of the dry ingredients to the batter. Mix, on low, only until the flour is absorbed. Add half of the sour cream. Mix on low to combine, then scrape down the bowl. Repeat this step until you've incorporated all of the flour and sour cream.
Scrape down your bowl and give the batter two stirs to finish blending. Pour your batter over the caramelized fruit.
Bake the cake for 60-65 min, or until a toothpick inserted into the middle of the cake comes out clean. If you find your cake is browning too much before the bake time is up, cover with a piece of aluminum foil.
Remove the cake from the oven and allow to cool for 5 minutes. Place an inverted plate over the cake pan and, carefully, flip the two over to remove the cake from the pan. Set the plate onto the counter with the pan still on top. Allow the pan to sit over the inverted cake for 5 minutes. This will allow the remaining sugar to coat the cake. Remove the cake, and allow to cool until warm.
This cake will keep for 4 days, covered, at room temperature. Enjoy!