Preheat oven to 180°C. Line a 6 or 8 inch round cake tin with baking paper. Bring a full kettle to the boil.
PREP CAKEPlace butter, cocoa powder and instant espresso powder in a medium bowl and pour over boiling water measure. Whisk until combined. In a second large bowl, combine flour, sugar and salt. Pour wet mix into dry mix and whisk to combine. Add egg and milk and fold to combine.
BAKE CAKETransfer mixture to cake tin and bake for 20-25 minutes, until a skewer inserted comes out clean.
PREP ICINGPlace butter, icing sugar, cocoa powder and milk in a large bowl. Use electric beaters to beat for about 6 minutes, until a smooth, spreadable consistency. Add extra milk if needed, 1 Tbsp at a time.
TO FINISHWait until cake has completely cooled before icing. Use a spoon to spread icing and decorate as desired.