Prepare the Cake:
Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.
In a mixing bowl, combine the devil's food cake mix, water, vegetable oil, and eggs. Beat with a hand mixer or stand mixer until well blended.
Pour half of the cake batter into the prepared baking pan and bake for 15 minutes.
Make the Caramel Layer:
While the first layer of cake is baking, melt the butter in a saucepan over medium heat.
Add the brown sugar, corn syrup, and sweetened condensed milk. Stir constantly until the mixture comes to a boil.
Continue to boil for 5 minutes, stirring constantly to prevent burning.
Remove from heat and stir in the vanilla extract.
Assemble the Cake:
Once the first layer of cake is baked, pour the caramel mixture evenly over the top.
Sprinkle the chopped pecans (if using) over the caramel layer.
Pour the remaining cake batter over the caramel and pecans.
Return the cake to the oven and bake for an additional 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Prepare the Chocolate Ganache:
In a small saucepan, heat the heavy cream until it just begins to simmer.
Remove from heat and add the chocolate chips. Let sit for 2-3 minutes, then whisk until smooth.
Finish the Cake:
Allow the cake to cool completely in the pan.
Once cooled, pour the chocolate ganache over the top of the cake, spreading it evenly with a spatula.
Let the ganache set before serving.