Preheat oven to 180 deg .Line the cupcake tray with paper cases.In a large bowl combine together all purpose flour,cocoa powder,baking powder,baking soda,salt,brown sugar and granulated sugar.Mix well using a fork or a whisk until well combined. Now add oil,butter milk,egg,vanilla extract and mix gently ,just until combined.
Next add the instant espresso powder into the boiling water and mix well until dissolved.Add this to the batter and mix gently until combined.(Do not over mix).The batter will be very thin.
Spoon the batter into the paper cases .Fill about 2/3 rd full.Bake for about 18 to 20 minuites or until a skewer inserted into the center of a cupcake comes out clean.Let cool in the pan for 10 minutes and then transfer onto a wire rack to cool completely.
To prepare the chocolate buttercream frosting ,cream the butter and powdered sugar until light and fluffy using a whisk or a hand blender.
Add the cocoa powder and whisk well until combined.Scrape down the sides of the bowl as needed.
Add the heavy cream ,vanilla extract and salt and mix well until combined.Transfer the froasting to a piping bag and pipe onto the cooled cupcakes.
