Preheat your oven to 350°F. Grease a 9×13 inch baking dish.
In a small saucepan, melt the butter and brown sugar over medium heat. Stir until combined.
Pour the mixture into the bottom of the baking dish and sprinkle chopped pecans evenly over the top.
In a large bowl, mix together cake mix, oil, eggs, sour cream, and water until well combined.
Pour the batter over the pecan mixture in the baking dish and smooth the top.
Bake for 40–45 minutes, or until a toothpick inserted in the center comes out clean.
Let the cake cool for 10 minutes, then carefully invert onto a serving platter.
Serve warm or at room temperature.