Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in the vanilla.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add to the creamed mixture, alternating with the buttermilk, beginning and ending with the dry ingredients.
Pour batter into the prepared pan. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
While the cake is baking, prepare the caramel topping. In a saucepan over medium heat, combine the brown sugar and butter. Cook, stirring constantly, until the sugar is dissolved and the mixture is smooth.
Remove from heat and stir in the heavy cream. Pour the caramel topping over the warm cake and let it cool completely.
Notes
Store leftover cake in an airtight container at room temperature for up to 3 days.