Lightly grease and fully line a 20 cm (8 in) square cake tin with baking parchment.

In a large heavy-based pan, add double cream, milk, butter, caster sugar, and light muscovado sugar.

Place the pan over low heat and stir constantly until the butter melts and sugars dissolve completely.

Increase the heat slightly so the mixture comes to a gentle boil and keep stirring continuously.

Simmer while stirring until the mixture reaches 116°C (241°F), the soft ball stage on a candy thermometer.

Remove from heat and let the mixture cool undisturbed to 110°C (230°F).

Stir in vanilla extract and vigorously beat the mixture until it thickens and loses its glossy shine.

Pour the thickened mixture into the prepared tin, smoothing it evenly into the corners.

Leave to cool and set at room temperature for 2 to 3 hours until firm.

Once set, remove from tin and cut into bite-sized pieces for serving.