Preheat your oven to 350℉. Grease your jelly roll pan.
In one bowl, mix together the flour, sugar, baking soda, and cinnamon. Set this aside.
In another bowl, whisk together the eggs and sour cream.
In a medium saucepan, bring the water, butter, and cocoa powder to a boil, then turn off the heat. Whisk this until it's smooth, then add and stir in the vanilla extract.
Add the sour cream and egg mixture to the flour mixture and stir together. I did it by hand; a hand mixer or stand mixer works, but you want to avoid over-mixing.
Add the warm chocolate mixture to the flour mix and mix together until it's all combined and smooth.
Pour your batter into the jelly roll pan and bake for about 18 minutes, or until a toothpick inserted comes out clean.
While the cake is baking, use another sauce pan to make the frosting. Bring the milk, butter, and cocoa powder to a boil.
Turn the heat off and mix in the powdered sugar. Once that mixture is smooth, stir in half of the pecans.
Keep this frosting warm and liquid until the cake is done baking. When the cake is done, remove it from the oven and pour the warm frosting over the top of it. Excess frosting may spill over the sides, so be prepared for that if necessary.
Once that's done, let the cake and frosting cool, and serve.