**Make the Cake:**
- Preheat oven to 350°F (175°C). Grease and flour two or three 9-inch round cake pans.
- In a large bowl, whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt.
- Add milk, vegetable oil, eggs, and vanilla extract. Mix until smooth.
- Slowly stir in boiling water until the batter is thin and well-combined.
- Divide the batter evenly between the prepared pans. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let cakes cool in pans for 10 minutes, then transfer to a cooling rack to cool completely.
**Prepare the Caramel Sauce:**
- Heat sugar in a saucepan over medium heat, stirring constantly, until it melts into a golden brown liquid.
- Add butter and stir until fully combined. Gradually pour in heavy cream, stirring constantly. Cook for 1-2 minutes, then remove from heat and stir in salt. Let cool slightly.
**Make the Ganache:**
- Heat heavy cream in a small saucepan until it begins to simmer. Remove from heat and pour over chocolate chips in a bowl. Let sit for 2-3 minutes, then stir until smooth and glossy.
**Assemble the Cake:**
- Place the first cake layer on a serving plate. Spread a layer of caramel sauce on top, followed by a sprinkle of chopped pecans. Repeat with the second layer.
- If using a third layer, add the final layer of cake and spread ganache over the top and sides. Drizzle remaining caramel sauce over the top and garnish with more pecans.
**Serve:**
- Let the cake set for 20-30 minutes before slicing.