Preheat oven to 350 F and line a 24 cup muffin pan with paper liners. In a large bowl whisk flour

granulated sugar

baking soda

and salt until combined. In a saucepan over medium heat melt 1 cup butter

then whisk in water and 5 tbsp cocoa powder and bring just to a simmer. Pour the hot cocoa mixture over the dry ingredients and stir until mostly smooth. Whisk in eggs one at a time

then mix in sour cream and 1 tbsp vanilla until the batter is smooth. Divide batter evenly into liners filling each about two thirds full. Bake until a toothpick comes out clean

15 to 18 minutes

then cool in the pan 5 minutes and transfer to a rack.

In a saucepan melt 1/2 cup butter over medium heat

whisk in milk and 3 tbsp cocoa powder and heat until hot but not boiling. Remove from heat and whisk in vanilla and a pinch of salt. Whisk in powdered sugar in batches until glossy and pourable

adding a splash more milk if too thick.

Spoon warm frosting over slightly warm cupcakes so it sets with a sheen

then sprinkle with pecans or flaky salt if using and let stand 20 to 30 minutes to set.