Preheat the oven to 180°C or 350°F
Line a muffin tray with 6 paper liners
Melt the dark chocolate and butter together in the microwave. Start with 30 seconds, then repeat in 10 second increments, stirring in between until smooth. It should take 50-60 seconds. Allow to cool a little
Into a mixing bowl, sift the flour, baking powder and cocoa.
Add a pinch of salt, along with the sugar and half of chocolate chips, then stir with a spatula to combine, making a well in the centre. (the rest of the chips can go on the top once the mixture is scooped)
In a small jug, combine the buttermilk, egg, vanilla and espresso.
Add this to the dry ingredients, then add the cooled melted chocolate and butter.
Using a rubber spatula, fold the ingredients together, mixing until only just combined.
Using a large ice cream scoop, divide the batter into the 6 liners
Scatter the remaining chocolate chips over the muffin mixture
Bake for 20 minutes
Cool in pan for 5 minutes, then serve