Preheat oven to 160c or 140c fan. Lightly oil an 8 x 28 cm slice tin, and line with non stick baking paper, overhanging the two long sides.
Combine the butter, sugar and golden syrup in a small saucepan, and stir over low heat until melted and smooth. Stir in vanilla essence.
Combine dry ingredients in a large bowl, make a well in centre. Pour in butter mixture, stir well. Transfer to prepared tin. Gently press with lightly oiled back of a spoon to form an even layer.
Bake slice for 25 minutes, until lightly browned. Cool in tin, then lift out and cut into bars. Place melted chocolate into a small zip lock bag, and snip off a corner. Drizzle over bars, leave to set.