To make buttercream beat butter and hazelnut spread. Add cocoa powder, powdered sugar, vanilla extract and add 1 Tablespoon of milk at a time until you reach your desired consistency. You will want a looser consistency than you would have for cupcakes.
To assemble, center your first layer of baked cake on your cake plate, then frost the top of that cake...then put another layer of cake...then another layer of frosting...then cake...then frost the entire cake! Use chopped hazelnuts to decorate.