Preheat your oven to 350 degrees F. Grease a sheet pan with non-stick cooking spray or butter. (It's optional, but I recommend lining the sheet pan with parchment paper too.)
In a large mixing bowl, stir together the flour, sugar, baking soda, salt, and cinnamon. Set aside.
In a small mixing bowl, whisk together the eggs and buttermilk. Set aside.
Add the water, butter, cocoa powder, and vanilla to a small saucepan. Place over medium heat and cook until the butter has melted, stirring constantly. Once you see steam you can turn off the heat and just let the residual heat melt everything together.
Pour the chocolate mixture into the flour bowl and mix until smooth and just combined. (Don't overmix!) Pour in the egg mixture and fold until you have a smooth batter.
Pour the cake batter into the prepared sheet pan and spread it into an even layer. Bake for 20 minutes or until a toothpick comes out clean.
Remove from the oven and set on a wire rack while you make the frosting.
Frosting
Sift the powdered sugar and cocoa powder together. Set aside.
Add the butter, milk, and vanilla to a small saucepan. Place over medium-high heat and bring to a boil, whisking occasionally.
Pour in the cocoa powder mixture and whisk to combine. Pour over the warm cake immediately and then spread edge to edge. (see note)
Sprinkle the toasted pecans and chocolate chips evenly over the cake. Then drizzle the entire cake with caramel sauce before cutting into 15 slices.
Notes
If you don't have buttermilk on hand, you can substitute sour milk instead. To make sour milk: Stir together 1 cup of milk with 1 tablespoon of lemon juice or white vinegar. Let sit for 5 minutes before using.
To toast the pecans: Place the pecans in a skillet over medium heat. Toast for 2-5 minutes until slightly darker and fragrant. Remove from the skillet to cool.
If you want a fudgy frosting vs a smoother spreadable frosting, add the cocoa into the pot with the butter and milk. Add the powdered sugar once everything is melted together. Pour over the cake and work VERY quickly to spread the frosting.