Roll out the shortcrust pastry on a parchment covered tart baking pan. Blind bake (cover with extra parchment and dry beans) in a preheated oven to 350F(180C) for 10 mins. Then spread nutella on top.
Put chocolate and butter in 2 bowls. Microwave until melted. Allow to cool a bit. Lightly whisk the chocolate with egg, then fold 1 yolk at a time until fully incorporated. Add sugar in.
Add sugar and orange zest in, fold warm butter in and mix until just combined. Fill the tart shell with the ganache and top with almonds. Bake for 11 mins.
Allow to cool 20mins then sprinkle powdered sugar and extra almonds on top and serve.