Pre-heat the oven to 200ºC (400ºF) and prepare a muffin tin. Melt the butter for the topping and set to one side for a while. Using a food processor for convenience, chop up the chocolate and hazelnuts until they are very fine. Add these to the melted butter along with the sugar. Stir to combine. Set to one side.
For the muffins: melt the butter and allow to cool. In the same food processor (give it a rinse round) add the butter and sugar and work for a minute or two. Add the egg, continue to work for a further minute and finally, add the milk and give it a quick whizz just to ensure everything is evenly combined.
Add the flour, baking powder, baking soda and vanilla sugar and on the lowest speed, work until evenly combined. The batter starts off runny but as with most cakes including GF flours, it does thicken so don’t panic.
Pour into ten cupcake or muffin holes almost to the very top. Sprinkle over the topping and bake for around 15-17 minutes (depending on how large your muffin cases are). Remove from the oven, allow to cool for a while, then enjoy while still warm!