Mix 6 egg yolks, 1/2 cup sugar, 1 Tbsp vanilla extract, 1/4 tsp salt with a whisk until smooth.

Add 4 Tbsp flour and whisk until smooth again. 
Pour milk 1/2 cup at a time while continuously whisking. 
Cook over medium heat, whisking non stop to prevent clumps from forming and the bottom from scorching.

Add 1/2 cup Nutella into the warm cream and whisk until it is well incorporated. Allow to completely cool to room temperature. Do no refrigerate. If you do, bring to room temperature before adding to the butter. {Tip: To speed up the cooling process you can spread the cream in a large baking pan, covering with plastic wrap touching the cream. }

Whip 1.5 sticks , unsalted, room temperature butter with a mixer on high speed for about 5 minutes, stopping 3-4 times to scrape down the bottom.

Add the cooled to room temperature custard into the whipped butter in thirds, whipping until almost fully incorporated before making the next addition. Stop and scrape the bottom of the bowl to make sure there are no unmixed chunks of butter.

Refrigerate cream until ready to use. Bring to room temperature before using in cakes or recipes.