Line 9 x 9-in (23 x 23-cm) square pan with parchment paper, allowing for overhang. Place sweetened condensed milk, brown sugar, butter, cinnamon, ginger, nutmeg and salt in large microwaveable bowl. Microwave on HIGH about 8 to 10 min., whisking every 2 min., until mixture has thickened and caramelized. To check for doneness, drizzle spoonful of mixture onto a plate, allow to cool and pinch together. If it is flexible but firm enough to hold its shape, continue to step 2. Otherwise, continue to microwave in 2-minute intervals until spoon test yields a fudge-like consistency.
Using oven mitts to handle the hot bowl, remove from microwave and stir in vanilla extract. Scrape mixture into prepared pan, smoothing top with a spatula or the back of a spoon. Cool completely at room temperature. Use overhanging parchment, lift fudge from the pan before slicing into 25 pieces.
To make festive boozy gingerbread fudge, add 1 tbsp (15 mL) brandy along with the vanilla extract.