heat oven to 350 F and grease a 13x18 inch sheet pan or jelly roll pan
whisk flour sugar baking soda and salt in a large bowl
bring butter water and cocoa to a boil in a saucepan then remove from heat
pour hot cocoa mixture into dry ingredients and stir until smooth
whisk in eggs sour cream and vanilla until fully combined
spread batter evenly in pan and bake until set and a toothpick comes out with moist crumbs about 16 to 20 minutes.
Frost and Set
during the last few minutes of baking heat butter milk and cocoa in a saucepan until steaming and smooth then remove from heat
whisk in powdered sugar and vanilla until glossy and pourable then stir in pecans if using
pour frosting over hot cake and spread quickly to the edges
cool 30 to 60 minutes until frosting is set then cut into bars.
Notes
1. Use buttermilk in place of sour cream for a slightly tangier cake and thinner batter. Toast pecans for deeper flavor or omit for nut free. For clean cuts chill 15 minutes after cooling and wipe knife between slices