Preheat oven to 350°F. Grease two 9-inch pans and line with parchment.
Whisk flour, sugar, cocoa, baking powder, baking soda, and salt.
Add eggs, milk, oil, and vanilla. Beat 2 minutes. Stir in boiling water until smooth.
Divide batter between pans. Bake 30–35 minutes. Cool 10 minutes, then transfer to racks.
Heat 1 1/4 cups cream until simmering. Pour over chopped chocolate. Stir until glossy ganache. Chill until spreadable.
Make caramel: melt sugar in a heavy pan without stirring until amber. Add butter, then slowly whisk in warmed 1/2 cup cream. Stir in sea salt. Cool slightly.
Stack cakes with ganache between layers. Spread ganache over top and sides.
Spoon cooled caramel over top, letting it drip. Chill 30 minutes before serving.