Combine the sugar, milk, glucose syrup, invert sugar, and cream in a saucepan. Cook to 115°C/239°F, stirring constantly.
Pour the hot mixture onto a marble slab. Place the fondant, chocolate liquor, and vanilla extract on top of the syrup.
Allow the mixture to cool to 50°C/120°F. Using a scraper, agitate until the mixture turns creamy and opaque, about 8 minutes.
Deposit into a frame 12 × 12 × ½in set on lightly oiled parchment paper.
Place another piece of lightly oiled parchment paper on top of the fudge. Using a rolling pin, flatten the top. Allow the fudge to crystallize before removing the parchment paper. Or, if desired, leave rippled for a more natural appearance. Crystallization will take about 30 minutes.
Cut into 25-mm/1-in squares. Wrap to prevent drying.