Preheat oven the 180°C/ 160°C Fan/Gas Mark 4. Line a 12 hole muffin tray with cupcake cases.
Place the flour, Baking Powder, Bicarbonate of Soda, Cocoa Powder, sugar and ground hazelnuts in a large bowl and mix together. Make a well in the centre.
In a separate bowl mix together the eggs, milk, oil and Vanilla Extract. Pour the wet mixture into the well in the centre of the dry ingredients and gently begin the fold together. Mix together until all combined and your mixture is smooth.
Divide the mixture evenly between the 12 cupcakes cases. Bake the cupcakes for 15 -20 minutes until risen and when a skewer is inserted into the centre of a cupcake it comes out clean. Leave to cool in the tin for 10 minutes and then remove from the tins and place on a wire rack to cool completely.
Once the cupcakes are cooled, take a sharp knife and cut a circle out of the top of each cake at an angle about 1cm (½ inch) from the edge. Chop the middle off the circle of sponge to it is flat. Fill the hole with a tsp of Nutella and place the sponge back on top.
Place the Vanilla Buttercream in a bowl and add Nutella and mix until smooth.
Place the buttercream in a piping bag fitted with a star nozzle. Pipe a buttercream swirl onto the top of each cupcake. To finish the cupcakes add a sprinkling of crushed hazelnuts!