Preheat oven to 350º.
In a round cake pan, melt 2 Tbsp. butter in the oven. Using a hot pad, tilt pan to coat bottom and sides with butter. Set aside until after cake batter is made.
Zest and slice orange. Sprinkle half of the zest in the cake pan. Set aside the other half for the batter. Remove peel from the orange slices.
Add dry ingredients and turn mixer on low. Then add in sour cream and remaining orange zest, and mix until just combined.
Sprinkle the cake pan with the reserved 1/4 cup brown sugar. Place sliced oranges over the brown sugar. Sprinkle cranberries into the pan, trying to get them mostly between the orange slices.
Spoon the batter over the fruit, and spread out until it covers them evenly.
Bake for 35 minutes, or until a toothpick inserted in the center comes out clean.