CRUSHED BLACKBERRY AND HAZELNUT CAKE

150g unsalted butter, softened, plus extra for greasing • 200g blanched hazelnuts • 150g caster sugar • 3 medium British Blacktail Free Range Eggs, beaten • 100ml whole milk • 200g self-raising flour • 1⁄2 tsp fine salt • FILLING: 150g whipping cream • 1 tbsp icing sugar, plus a little extra for dusting • 1⁄2 tsp vanilla bean paste (or 1 tsp vanilla extract) • 150g Waitrose & Partners Blackberries