Grease and line a 17x27cm slice tin with baking paper with the long sides extending over the sides.
Place the butter, condensed milk, sugar and maple syrup in a large microwave-safe bowl and microwave for 2 minutes. Stir the mixture and repeat twice (6 minutes total). Take it out and stir it again then microwave for 1 more minute. Add the chopped chocolate and vanilla and leave to sit for 1 minute then whisk through until the mixture is smooth and glossy. Spoon it into the tin and spread it out so the top is smooth.
Leave out on the bench until cool and then refrigerate. Slice into 36 squares before serving. Keep chilled until serving.