Iced Ring Biscuits

Dough

• 50 g caster sugar
• 100 g unsalted butter, diced
• 1 egg yolk, from medium egg
• 1 tsp natural vanilla extract
• 150 g plain flour, plus extra for dusting
• 25 g cornflour

Icing

• 160 g icing sugar
• 3 Tbsp water, hot
• food colouring pastes, as desired