Heat the butter, nutella and peanut butter in a small saucepan over low heat until melted. Whisk until smooth. Add in the chocolate and let it sit for a minute, then whisk until it melts. Stir in the sugar, salt and vanilla extract. Beat in the eggs one at a time, beating until combined. Stir in the flour until JUST combined. Pour the batter in the greased tart pan. Bake for 25 to 30 minutes. It won’t appear finished, but you don’t want to bake it longer! Let it cool completely.