Spoon into the bundt pan. Bake 40-50 minutes, or until a toothpick inserted into the cake comes out clean. Let cake cool 10 minutes in the pan, then invert onto a wire rack to cool completely.
To make the topping, combine butter, dark brown sugar, and cream in a small saucepan over medium heat and melt, stirring. Once melted, increase slightly and bring to a full rolling boil for 1 minute.
5 tablespoons unsalted butter, 1/2 cup packed dark brown sugar, 1/3 cup heavy cream
Remove from heat and whisk in vanilla (be careful, as the caramel will bubble up). Add sifted powdered sugar and whisk until completely incorporated and smooth. Whisk in salt to taste. Let cool slightly, then pour over the cake. The topping will firm up as it cools.
1/2 teaspoon pure vanilla extract, 2/3 cup powdered sugar, 1/2 - 1 teaspoon fine sea salt