Preheat the oven to 180°C (fan 160°C)/350°F/gas mark 4. Line a 12-cup muffin tin with cupcake papers.
Combine the flour, cocoa, chai mix, baking powder and salt in a bowl, and mix together.
In a large mixing bowl, cream the butter and sugar together, using an electric hand whisk on medium speed. This will take about 3 minutes. Add the eggs one at a time and beat in well. Then stir in the vanilla.
With the whisk on low speed, alternately add the flour mixture and milk to the creamed butter mixture and beat until well combined and smooth.
Fill the cupcake cups with this batter up to three-quarters full. Bake in the preheated oven for about 15 minutes or until risen, and an inserted skewer comes out clean. Let the cupcakes cool completely on a wire rack before frosting.
For the frosting, combine all the ingredients in a bowl and mix with a wooden spoon until smooth. Refrigerate until the cakes have cooled.
Spread or pipe the frosting on top of the cupcakes and, if you like, sprinkle with extra cocoa powder and icing sugar to decorate.