Preheat the oven to 160℃/140℃ fan/gas 3. Lightly grease a deep 20cm spring form cake tin with sunflower oil and line the base and sides with baking paper.
Sift the flour, cocoa powder, bicarbonate of soda, and baking powder into a large bowl. Add the sugar and mix well. Make a well in the centre of the dry ingredients and add the golden syrup, eggs, sunflower oil, and milk. Beat with an electric whisk until smooth and then pour into the prepared tin.
Bake for around 45 minutes and have a peek in the oven window. The cake should have a crust on top - if it’s catching, quickly open the oven and cover it in a bit of tin foil. Bake for a further 20 minutes or until a skewer inserted comes out clean. Allow to stand for about 10 minutes before removing from the tin and cooling completely on a wire rack.
To make the buttercream, mix together the cocoa powder with 4 tablespoons of hot water. Allow to cool slightly before adding the butter and whisking for a few minutes to combine.
Gradually add the icing sugar, beating until fully combined. Continue to beat on a medium speed for 5 to 10 minutes. You should see it become paler and fluffier, and when you taste it, it should be much smoother and less grainy.
Cut the cake into two even layers using a serrated knife. Spread under a third of the butter cream onto the bottom layer and place the top layer on top. Spread a thin layer over the cake to seal the crumbs and set in the fridge for an hour or in the freezer for thirty minutes until dry to the touch.
Spread another thick layer all over the cake and smooth - you might want to reserve a few tablespoons of icing for piping a decoration around the edge. Alternatively, you can decorate with chopped chocolate or a dusting of cocoa powder - get creative!
Serve with a cup of tea or a coffee and enjoy all of your hard work! The cake will last about half a week as it’s a very moist sponge.