Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans. Line the bottoms with parchment paper.
In a large bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, baking powder, and salt. Make sure there are no lumps.
In a separate medium bowl, whisk together the eggs, buttermilk, vegetable oil, and vanilla extract until well combined.
Add the wet ingredients to the dry ingredients and mix on low speed (or with a whisk) until just combined. Be careful not to overmix.
Slowly pour in the hot water or coffee while mixing on low speed until the batter is smooth and thin. This will create a very moist cake.
Divide the batter evenly between the prepared cake pans.
To make the caramel swirl: In a medium saucepan over medium heat, combine the brown sugar, butter, and heavy cream. Stir constantly until the butter is melted and the sugar is dissolved. Bring to a gentle boil and cook for 2-3 minutes, stirring, until slightly thickened. Remove from heat, stir in vanilla extract and salt. Let it cool slightly.
Drizzle about half of the slightly cooled caramel over the batter in each cake pan. Use a knife or skewer to gently swirl the caramel into the batter, creating a marbled effect.
Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes before inverting them onto a wire rack to cool completely.
While the cakes cool, prepare the milk chocolate frosting. In a large bowl, cream the softened butter until fluffy. Beat in the cocoa powder.
Gradually add the powdered sugar, alternating with the milk, and beating until smooth and creamy. Stir in the vanilla extract.
Once the cakes are completely cool, place one layer on your serving plate. Spread a generous amount of frosting over the top. Sprinkle half of the toasted pecans over the frosting.
Place the second cake layer on top. Frost the entire cake – top and sides – with the remaining milk chocolate frosting. Decorate the sides and top with the remaining toasted pecans.