Grease and line a 20cm square baking tin with baking parchment. Put the sugar, condensed milk and butter in a large heatproof bowl (the mixture will bubble up as it cooks to roughly double its size), add a good pinch of salt and stir together.
Heat the mixture in the microwave on High for 10 mins, stirring every 2-3 mins. Keep an eye on the mixture, as it will bubble up and may go over the sides of the bowl.
Add the vanilla and liqueur to the mixture, whisk together and heat on High for another 2 mins. The mixture should be caramel-coloured and thickened. Scrape into the tin and set aside for 45 mins to 1 hr until cool and firm before cutting. Pack into a box and eat within 3 weeks.