Preheat oven to 150C. Grease deep 20cm round cake pan; line base with baking paper.
Stir Nutella spread, butter, sugar and the water in medium saucepan over low heat until smooth. Cool 15 minutes. Stir in sifted flours, ground hazelnut, eggs and rum.
Pour mixture into pan; bake for about 1 hour 30 minutes. Stand cake in pan 5 minutes, turn, top-side up, onto a wire rack to cool.
Meanwhile, make fudge frosting. Spread cake with frosting and chill it for 2 hours before consuming.
Fudge frosting - Stir butter, the water and brown sugar in small saucepan over low heat until sugar dissolves. Remove from heat, stir in rum.Sift icing sugar and cocoa powder into small bowl; gradually stir in hot butter mixture until smooth. Cover; refrigerator about 10 minutes or until frosting thickens. Beat frosting with a rubber spatula until spreadable.
Recipe by Anncoo Journal at https://www.anncoojournal.com/nutella-mud-cake-with-fudge-frosting/