Preheat oven to 200°C (180°C fan-forced) and line a 12-hole (1/3 cup capacity) muffin pan with paper cases.
In a large mixing bowl place flour, cocoa, baking powder, salt, sugar and chocolate together, stirring until combined.
In a large jug, combine eggs, buttermilk, sour cream, vanilla, coffee and oil, whisking until well combined. Pour over the dry ingredients, stirring until just combined. Do not overmix, the batter should be thick and lumpy.
Evenly divide mixture between prepared muffin cases and place muffin pan in preheated oven for 20 minutes or until muffins are cooked through.