VANILLA POACHED PLUMS, CARDAMOM CUSTARD & HAZELNUT-SUMAC CRUMBLE

• 500ml whole milk • 5 cardamom pods, seeds shaken out and crushed • 1 vanilla pod • 250g caster sugar • 8-10 plums • 3 eggs • 50g plain flour | Crumble • 100g plain flour • 50g caster sugar • 50g unsalted butter, diced and chilled • 25g hazelnuts, roughly chopped • ½ tbsp sumac • ¼ tsp sea salt flakes