Pre-heat your oven to 200C/180C Fan/Gas Mark 6. Line an 8" Square Pan with greaseproof paper.
In a small bowl, break up the dark chocolate and melt in the microwave in 20 second intervals, stirring in between to make sure it doesn't burn.
In a large mixing bowl, whisk together melted butter and both sugars. With an electric/stand mixer, go for a solid 3-5minutes so it is totally combined. Add in the eggs and vanilla and combine again.
Now, add the chocolate slowly (check it first, to make sure it isn't so hot that it will cook the eggs) and combine once again.
Add the flour and cocoa powder, plus a pinch of salt. Fold gently into the wet mixture with a rubber spatula or wooden spoon until just combined. Fold in the chopped chocolate.
Pour the batter into the lined pan and bake for 30 minutes. Let cool on a wire rack before making the frosting.
Add the butter and chocolate hazelnut spread to a large bowl and mix on high speed until soft and combined. Add the cocoa powder and half the icing sugar and whisk on high speed until combined. Add the remaining icing sugar - until you reach the desired consistency. Whisk on high for a few minutes, until the frosting looks fluffy! Spread over, top with any decorations before slicing and serving.