Preheat the oven to 350 degrees F. Lightly grease a 9X13-inch pan with nonstick cooking spray (I use metal for more even baking; if using a glass pan, turn the oven down to 325 degrees F). You can also line the pan with parchment paper, optional.
For the cake: in a large mixing bowl, combine the butter, granulated sugar, egg, buttermilk, baking soda, salt and flour. Mix until no dry streaks remain and dough is mostly combined. Add the cranberries and stir with a rubber spatula until evenly combined into the batter.
Spread the batter evenly in the prepared pan. Sprinkle evenly with the brown sugar and pecans. Bake for 45-55 minutes until the cake is set and a toothpick inserted in the center comes out clean or with moist crumbs.
For the sauce: combine the butter, cream and sugar in a small saucepan and heat over medium until the mixture comes to a gentle simmer. Cook for 1-2 minutes. Remove from the heat and stir in the vanilla. Keep warm. (Or refrigerate and warm later when serving.)
Cut the cake in pieces and drizzle with warm vanilla sauce. It's also very delicious served with a scoop of vanilla ice cream.
Cranberries: I haven't tried this recipe with frozen cranberries but I'm guessing it would work (don't thaw first). It will probably also transition really well to other fruits like blueberries or raspberries. The addition of a little fresh lemon zest in the batter would be delicious.