PREP: Preheat the oven to 180C / 350F / 160C fan forced. Grease and line an 8 inch spring form tin with baking paper. Place an oven rack in the middle of the oven.
MELT: Place the butter and white chocolate into a medium bowl. Melt in the microwave in 30 second bursts, stirring really well between each (notes). Set aside to cool for a few minutes.
DRY INGREDIENTS: In a large bowl whisk together the flour, baking powder and salt. Add the sugar and whisk really well until there are no lumps left.
WET INGREDIENTS: Add the golden syrup, along with the milk, eggs and vanilla to the melted chocolate mixture. Whisk until combined and there are no clumps of egg white left.
MIX THE BATTER: Pour the wet ingredients into the dry then use a silicon spatula to mix them together until all fully incorporated (don't overmix).
BAKE: Pour the batter into the prepared tin and bake for 60-70 minutes, turning the tin at the halfway point, until a toothpick inserted comes out with just a crumb or two attached. (notes)
Let it cool 10-15 minutes in the tin before transferring to a wire rack to cool completely.
FOR THE ICING
Add the butter, sugar and milk to a small saucepan, stirring over medium heat. Remove the pan from the heat and cool for 10 minutes.
Once the butter has melted and the sugar dissolved, bring to the boil, then reduce the heat and simmer for 1 minute.
Add the icing sugar and salt and whisk in until fully incorporated.
Let the icing cool for 5-10 minutes before pouring it over the cooled cake.
Allow the icing to set for another 5-10 minutes, then use a speed peeler to grate chocolate curls over the top of the cake.
For best results, you should always weigh ingredients where a weight is the first measurement given. Kitchen scales are relatively cheap but if you can’t weigh the ingredients, use the spoon and level method (don’t scoop).
All ovens are different, one will bake slower or quicker than the next, so start checking for doneness with 10-15 minutes left.
Don't be tempted to speed up the melting of the chocolate and butter as you may end up burning the chocolate or coffee which will make it bitter. 30 second bursts and stir really well between each. It should take no longer than 2 minutes.
If you can't get golden syrup, you can also use rice malt syrup or maple syrup.