Method:

Heat the oven to 180°C/gas 4. Coat an 8-inch square cake pan with nonstick spray and line the bottom with parchment paper.
Beat the butter and sugars together in a large bowl with an electric mixer until fluffy. Beat in the egg, yolk, and vanilla until combined, scrape down the bowl, and then sprinkle the baking powder, baking soda, and salt over the batter and beat until very well combined. Add the buttermilk and mix to combine; the batter may look curdly, but this isn’t a problem. Add the cocoa and flour, and mix until the flour just disappears.
Scrape the batter into the prepared pan and smooth the top. Bake for 20 to 30 minutes, until a toothpick inserted into the center of the cake comes out batter-free. Let cool in the pan on a cooling rack for 10 minutes, then run a knife around the cake to ensure it is loose, flip it out onto a cooling rack, and then flip back right side up again onto another. Let cool completely before frosting. You can hasten this process along in your freezer, where it shouldn’t take more than 15 minutes to fully cool.

Make the frosting in a food processor. Place the frosting ingredients in a food professor and run the machine to mix. Scrape down the bowl, then process for another 1 to 2 minutes just until smooth and somewhat fluffed.
Make the frosting with a hand or stand mixer. Beat the butter, icing sugar, and salt, if using, in a large bowl with electric beaters (for a hand mixer) or the paddle attachment, not the whisk (for a stand mixer), until fluffy. Pour in the chocolate, cream or milk, and vanilla, and beat until combined, then 1 more minute to whip it further.
Decorate: Top with Raspberries and Maltesers or Chuckles. These are the favourites of the Dad who’s Fathers Day cake this is.