Preheat oven to 350°F (175°C). Grease and line two 8-inch round cake pans.
In a bowl, whisk flour, baking powder, baking soda, and salt.
Beat butter, granulated sugar, and brown sugar until light and fluffy.
Add eggs one at a time, mixing well. Stir in vanilla extract.
Add dry ingredients alternately with buttermilk, mixing gently until just combined.
Divide batter evenly between pans and bake for 25–30 minutes or until a toothpick comes out clean.
Cool cakes completely on a wire rack.
For frosting, melt butter and brown sugar in a saucepan. Add cream and simmer 2–3 minutes.
Remove from heat, add vanilla and salt, then beat in powdered sugar until smooth.
Frost the cooled cake layers evenly and assemble.