Brush six 3.5cm deep, 5 x 8cm loaf pans or an 18cm (base measurement) square cake pan with the melted butter to grease. Line the base and sides with non-stick baking paper, allowing the paper to overhang the sides.
Combine the sweetened condensed milk, brown sugar, butter and glucose in a 2-litre (8-cup), heatproof, microwave-safe glass jug or bowl. Cook, uncovered, on Medium-High/650watts/70%, using a balloon whisk to stir every 2 minutes, for 6-8 minutes or until the mixture begins to boil.
Continue to cook on Medium-High/650watts/70%, stirring every minute, for 3-4 minutes or until the mixture is thick, golden and boils rapidly. With a tea towel or oven mitt, very carefully place the jug on a board and set aside for 1 minute or until the bubbles subside.
Add the white chocolate and use a wooden spoon to stir continuously until the chocolate melts and the mixture is smooth. Stir in the walnuts and liqueur and mix well.
Spoon the mixture evenly into the prepared pans. Set aside for 2 hours to cool to room temperature and become firm. Remove from pan, wrap in greaseproof or non-stick baking paper and store in an airtight container in the fridge. Cut the fudge into 1cm-thick slices to serve.