Step 1

Preheat the oven to 350*F/ 180*C/ gas mark Butter an 8-inch square baking tin. Line with baking paper leaving an overhang on two sides. Set aside.

Step 2

Beat the sugar and eggs together in a bowl until very light and foamy. (It will look like it is thickening)  Beat in the sour milk, oil, melted butter, vanilla and salt until thoroughly emulsified. Whisk in the cocoa powder.

Step 3

Sift the flour, soda, and baking powder together. Whisk into the batter until well combined and smooth. Stir in the hot coffee/water.

Step 4

Pour the batter into the prepared baking tin and tap the bottom on the counter a few times to level it off and release any large air bubbles.

Step 5

Bake in the preheated oven for 35 to 45 minutes. It is done when well risen and a toothpick inserted in the center comes out clean. The top should also spring back when lightly touched.

Step 6

Set on a wire rack and allow to cool in the pan for 15 minutes before lifting out of the pan and onto the rack to cool completely.

Step 7

To make the frosting, measure all of the ingredients into a bowl and beat together with an electric hand mixer until you have a smooth, light and fluffy icing. It should be fluffy and not runny in the least. If you think yours is not fluffy enough, beat in additional icing sugar one heaped dessertspoonful at a time.

Step 8

Spoon onto the top of the cake and spread over in swoops and swirls.

Step 9

Store in an airtight container.