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Melt the butter and leave to cool for a while.
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Pre-heat the oven to 175°C (350ºF) and prepare a baking tin with grease-proof paper or foil.
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Mix the butter with the brown sugar and vanilla sugar until well combined.
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In a separate bowl, mix the oats, flour and baking soda. Add to the wet ingredients and stir until well mixed.
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Press around half of the mixture into the baking tin, keeping the remainder to use later. Bake for around 10 minutes until the edges are starting to brown.
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Remove from the oven and spread the caramel evenly over the top. Then sprinkle the chocolate over the top and lastly, cover with the retained oat mixture.
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Put back in the oven and bake for a further 15 minutes or so, until it is just browning on top and the caramel starts to bubble up a little.
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Allow to cool completely and then place in the fridge for several hours (preferably overnight if you can). Slice into squares and enjoy!