Preheat the oven to 350°F (175°C). Spray a 9 x 13 x 2-inch baking pan with nonstick spray and line it with parchment paper, leaving a 1-inch overhang for easy removal.
In a small saucepan, combine the unwrapped caramels and 1/3 cup evaporated milk. Heat over low, stirring constantly, until the caramels melt and the mixture is smooth, about 5 minutes. Remove from heat and let cool slightly.
In a large bowl, mix the cake mix, the remaining 1/3 cup evaporated milk, melted salted butter, and 2 tablespoons flour until fully combined.
Press half of the batter evenly into the bottom of the prepared pan using your fingers or a spatula. It may be thin; that is normal. Bake this layer for 6–8 minutes.
Remove the pan from the oven and immediately sprinkle the 12 ounces of semi-sweet chocolate chips evenly over the hot baked layer. Pour the slightly cooled caramel mixture evenly over the chocolate chips.
Crumble the remaining batter over the caramel layer in small pieces to cover as evenly as possible. If using, sprinkle the 1/2 cup chopped pecans on top.
Bake the assembled pan for an additional 18–20 minutes, until the top is set. Remove from oven and let cool completely so the caramel firms; chill briefly if you want to speed this up before slicing.