How to Make It
Preheat the oven to 350°. Butter and flour two 10-inch round cake pans and line the bottoms with parchment paper. In a bowl, whisk the 4 cups of flour, the baking powder, salt and baking soda. In the bowl of a stand mixer fitted with the paddle, beat the 2 sticks of butter with the sugar and vanilla seeds at medium speed until light and fluffy, 3 minutes. Beat in the eggs 1 at a time until incorporated, then beat until very pale and billowy, 3 minutes. At low speed, alternately beat in the dry ingredients and buttermilk until just combined.
Divide the batter between the prepared pans and bake for 35 minutes, until the cakes are golden and a toothpick inserted in the centers comes out clean. Transfer to a rack; let cool in the pans for 30 minutes. Unmold the cakes, peel off the parchment and let cool. Place 1 cake layer on a cake stand or serving platter.
In a large, heavy-bottomed saucepan, melt the butter over moderately high heat. Stir in the sugar, buttermilk, and baking soda. Cook, stirring constantly, until the mixture foams up and then subsides, 5 to 7 minutes. Continue cooking, stirring steadily, until the caramel mixture is very dark brown and reaches 240° on a candy thermometer, about 10 minutes. Carefully pour the mixture into the bowl of a stand mixer fitted with the paddle, add the vanilla and beat at low speed for 3 minutes, until thickened but still pourable.
Using a rubber spatula and working quickly, scrape about 1 cup of the frosting onto the cake layer on the stand and spread it to the edge. Top with the second cake layer. Pour the remaining frosting on top. Using an offset spatula, quickly spread the frosting over and around the cake to cover completely. Let the frosting cool for at least 2 hours before serving.