Pre-heat the oven to 350℉/170℃. Line a 9x13" sheet pan.
In a large bowl, or the bowl of a stand mixer, combine the flour, sugar, baking powder and baking soda.
Melt the butter with the cocoa powder (either in the microwave or in a small saucepan). Add the boiling water and stir until fully combined. If there are still any powder lumps remaining, whisk further or heat up further. Pour this mixture in the bowl with the flour mixture and combine.
Add the eggs, one at a time and beating in between. Finally, add the buttermilk and stir everything together. Pour into the lined pan and bake 20 minutes, until a skewer inserted comes out clean.
While the cake is baking, make the glaze. To a medium/large saucepan, add the butter, cocoa powder and milk and heat while stirring until the butter is melted and everything is combined. Add the powdered sugar and whisk until a glaze forms and no lumps remain. Remove from the heat, stir in the pecans and pour this glaze over the cake while it is still warm.
Let everything cool together until set, then slice and serve!