Preheat the oven to 350°F (180°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, sift together flour, cocoa powder, sugar, baking powder, baking soda, and salt.
Add milk, vegetable oil, eggs, and vanilla extract to the dry ingredients. Mix until well combined.
Stir in the boiling water to the batter. The batter will be thin, but that’s okay.
Pour the batter evenly into the prepared cake pans. Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.
To make the salted caramel sauce, melt butter in a saucepan over medium heat. Stir in brown sugar and heavy cream. Bring the mixture to a simmer and cook for 2 minutes while stirring constantly.
Remove the caramel sauce from heat and stir in sea salt flakes. Let it cool slightly.
Place one cake layer on a serving plate. Spread a layer of salted caramel sauce on top of the cake. Sprinkle with chocolate chips.
Place the second cake layer on top and repeat the process with caramel sauce and chocolate chips.
Drizzle the top of the cake with the remaining salted caramel sauce and sprinkle with additional sea salt flakes.
Refrigerate the cake for at least 1 hour before serving to allow the flavors to meld together.