1. Melt the butter in a medium heavy-based saucepan. Add the brown sugar and cook for 2-3 minutes, stirring occasionally, until the sugar has dissolved. Add the cream and bring to a simmer, stirring until you have a nice thick butterscotch sauce.
2. Meanwhile, place the cornflour in a bowl with the salt and slate with a couple of tablespoons of the milk until smooth, then whisk into the rest of the milk in a large bowl. Whisk in the egg yolks with the vanilla extract and then gradually add the hot butterscotch sauce, whisking constantly.
3. Wipe out the saucepan. Strain the mixture back through a fine-mesh sieve back into the saucepan and cook over a medium heat for 5-6 minutes, stirring regularly, until the butterscotch custard starts to thicken.
4. Remove from the heat and stir in the vodka. Pour into pretty glasses or ramekins and chill for at least an hour or up to 3 days.
5. For a lighter result, allow the custard to cool in a bowl with the surface covered with cling film to prevent a skin forming. To serve, whip lightly with a whisk or use the paddle of a kitchen aid before spooning into the glasses or ramekins. Garnish with crushed honeycomb.