Pre-heat the oven to 180ºc and line a rectangular sheet pan/cake pan with baking paper.
In a large bowl, whisk together all the dry ingredients.
Add the remaining ingredients and mix at medium speed for 2 minutes until well combined, scraping down halfway to make sure all the ingredients are incorporated.
Transfer the batter to the prepared cake pan and place in the oven.
Allow to bake for 35-40 minutes or until a skewer inserted comes out clean.
Remove from the oven and allow to cool completely before frosting.
To make the frosting, beat together the butter and peanut butter.
Add the icing sugar, cocoa powder, coffee and milk and beat until thick and smooth.
While mixing, slowly pour in the chocolate and allow to mix into the frosting. Mix for a further 1-2 minutes until the frosting is whipped and well combined.
Spread the frosting onto the cooled cake and decorate with sprinkles.