Grease a slice tin and line with baking paper, leaving excess on the sides to lift out slice once set.
Pre-heat oven to moderate (180 degrees celsius, fan-forced).
Combine butter and golden syrup in a saucepan. Stir gently over low heat until melted.
Mix bicarb soda with boiling water and add to the melted mixture. Wait for the mixture to froth and then combine with the dry ingredients.
Place about ⅔ of the mixture into the slice tin. Press into tin evenly using the back of a dessert spoon or fingertips.
Make the filling by combining the chocolate, condensed milk and butter in a saucepan and stirring over low heat until chocolate has melted, and then adding the vanilla extract and macadamias and stirring to combine.
Spread the filling evenly over the slice base.
Sprinkle teaspoonfuls of the remaining base mixture over the top of the filling.
Bake for 35 minutes or until the top has turned a golden brown colour.
Allow the slice to cool in the tin.
Remove from the tin using the overhanging baking paper and place into fridge for at least 2-3 hours before cutting into squares using a sharp knife.