Preheat oven to 450F. Generously butter two 4 ounce ramekins (or similar size, oven safe and small containers) and dust with unsweetened cocoa powder. Set aside.
In a small microwave-safe bowl, melt Nutella and butter (Note: I use the microwave for this step - heating in 15-30s increments between stirs).
In a medium sized bowl, whisk together egg, egg yolk, salt, and sugar until egg mixture is a slightly paler colour, and frothy (2-3 mins). Slowly mix warmed Nutella/butter mixture into the egg mixture.
Add 1 tsp of cocoa and a pinch of salt to the batter. Stir mixture together one final time and divide equally, into your two prepared ramekins.
Bake at 450F for ~11 min or until edges are set and the middle is still slightly sunken in/wobbly. Remove from oven and allow the lava cakes to cool for 3-5 min in the ramekins. (Note: ramekin will be hot - be sure to use a towel/oven mitt to handle the hot ramekins!).
Gently loosen the edges of each ramekin with a knife and invert onto a plate. After a few seconds, remove ramekin from lava cakes using a towel/oven mitt.
Decorate with a dusting of icing sugar, crushed hazelnuts, drizzled Nutella, berries, or ice-cream.
Serve hot and enjoy!