Chocolate and spearmint caramel slice

Name: Chocolate and spearmint caramel slice
Prep time: 15 min
Cook time: 1 h
Total time: 1 h 15 min
Cuisine: European, British
Ingredients: 220g shortbread biscuits; 50g unsalted butter, melted, cooled; Cocoa powder, to dust; 100ml pure (thin) cream; 3 drops peppermint essence; 1 cup spearmint leaves; 1/2 cup (110g) caster sugar; 40g unsalted butter; 200ml thickened cream; 30g unsalted butter, chopped; 2 tbs glucose syrup; 200g dark chocolate, chopped; 250g milk chocolate, chopped; 2 tbs sunflower oil