Chocolate Cake with Salted Caramel and White Chocolate Buttercream

1. Preheat a convection oven to 350 degrees F. Whisk the vinegar into the milk in a medium bowl and set it aside to curdle for 5 minutes.
2. Add the granulated sugar, oil and vanilla to the milk mixture and beat by hand with a whisk until foamy.
3. Sift together the flour, cocoa, baking soda, baking powder and salt in a large bowl. Add the dry ingredients to the wet ingredients in 2 batches, beating with an electric mixer fitted with a whisk attachment until no lumps.
4. Grease three 6-inch cake pans with nonstick cooking spray. Divide the batter among them and bake until a toothpick inserted in the center of each cake comes out clean, about 20 minutes
5. Let cool slightly, then remove the cakes from the pans. Cool the cakes in the refrigerator for 30 minutes. Soak the cake rounds with Coffee Simple Syrup by brushing about 1/3 cup onto each layer as you build the cake. To build: Spread the White Chocolate Buttercream on one soaked cake round, then add a heavy drizzle of Caramel Sauce. Add a second layer and repeat. Add the last layer and frost the whole cake with White Chocolate Buttercream.

Coffee Simple Syrup:

1. Add the sugar to a small saucepan fitted with a lid. Pour in the coffee and stir. Bring the mixture to a rolling boil over medium heat, stirring constantly, until sugar completely dissolves. Turn off the heat. Cover immediately and let cool for 15 minutes. 

White Chocolate Buttercream:

1. Put the white chocolate chips in a metal bowl. Bring the granulated sugar and 3 cups water to a boil in a large saucepan. Reduce the heat. Mix the cornstarch with 1 cup of water in a small bowl to combine. Stir it into the sugar mixture. Bring to a boil. Pour the sugar mixture over the white chocolate. Stir by hand with a whisk to combine. As mixture cools, stir 2 more times over the next 40 minutes. Let cool to room temperature.
2. Transfer to the bowl of an electric mixer fitted with a whisk attachment, add the butter and vanilla and mix until light and fluffy. 

Caramel Sauce:

1. Combine the light brown sugar, butter, evaporated milk, vanilla and salt in a small saucepan over medium heat. Cook, stirring, until thickened.