Preheat oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan.

1. In a large bowl, whisk together flour, sugar, baking soda, cinnamon, and salt.
2. In a saucepan over medium heat, combine butter, cocoa powder, and Dr Pepper. Bring just to a boil while stirring, then remove from heat.
3. Pour the warm Dr Pepper mixture into the dry ingredients and stir until just combined.
4. In a separate bowl, whisk together buttermilk, eggs, and vanilla extract. Stir into the batter until smooth.
5. Pour the batter into the prepared pan and spread evenly.
6. Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean.
7. While the cake bakes, prepare the frosting: In a saucepan, melt butter with cocoa powder and Dr Pepper over medium heat. Bring to a simmer.
8. Remove from heat and whisk in powdered sugar and vanilla until smooth. Stir in chopped pecans if using.
9. Pour the warm frosting evenly over the hot cake immediately after it comes out of the oven.
10. Let the cake cool slightly before slicing and serving.