Can be mixed together by hand, but a standing mixer works better.
Melt chocolate chips and allow to cool slightly.
In a small bowl put cocoa and espresso powder (if using) plus 4 tablespoons hot water to bloom the cocoa. Mix and let sit.
Mix shortening and let machine whip it until light. Add some of the powdered sugar and mix on low. Add the chocolate and mix on low. Add the cocoa powder mixture and mix on low. Add more powdered sugar and mix on low. Add 4 tablespoons of water and mix on low until things come together.
If this is enough water, once things come together turn the mixer up high to whip it into a lighter mix. Add more water, one or two tablespoons at a time until things are whip-able.
If mixing by hand you can simply put everything in a bowl and mix with a spoon. It will be less light and fluffy but it will a darker color.