Step 1: Preheat your oven to 350°F (175°C) and grease a 13x18 inch baking sheet.

Step 2: In a large bowl, whisk together the flour, sugar, baking soda, and salt. Set aside.

Step 3: In a saucepan, melt the butter over medium heat. Add the water and cocoa powder, stirring until smooth. Bring to a boil, then remove from heat.

Step 4: Pour the chocolate mixture into the dry ingredients, stirring until combined.

Step 5: Add the sour cream, eggs, and vanilla extract. Mix well until the batter is smooth.

Step 6: Pour the batter into the prepared baking sheet, spreading it evenly.

Step 7: Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted in the center comes out clean.

Step 8: While the cake is baking, prepare the frosting by melting butter in a saucepan. Stir in cocoa powder and milk, bringing to a boil. Remove from heat and add powdered sugar and vanilla, whisking until smooth.

Step 9: Pour the warm frosting over the warm cake and sprinkle with chopped pecans.

Step 10: Allow the cake to cool slightly before serving.