Preheat the oven to 350°F. Melt 2 tablespoons of the butter and pour it into a 9-inch springform pan.
In a medium bowl, combine the cranberries and brown sugar, then scatter the mixture on top of the butter in the pan. Sprinkle the pecans evenly on top of the cranberries.
In a medium bowl, use a hand mixer to beat the remaining 4 tablespoons of butter and sugar at medium speed until well blended. Beat in the eggs one at a time, and then beat in the vanilla.
In a small bowl, combine the flour, baking powder, and cinnamon and stir well. Beat a third of the dry ingredients into the creamed mixture, followed by half of the buttermilk. Continue alternating in this way until you’ve beaten in the last of the dry ingredients. Spoon the batter evenly over the cranberries.
Bake the cake until a wooden toothpick inserted in the center comes out clean, about 50 minutes. Let it cool in the pan for about 5 minutes, then invert the cake onto a serving platter and remove the pan.