Set a 12 × 12 × ½-inch confectionery frame on lightly oiled parchment. Prepare a second sheet of lightly oiled parchment and a rolling pin if you want a smooth top. Have a clean stainless-steel worktable and rigid plastic bowl scrapers ready.

Combine the granulated sugar, milk, glucose syrup, invert sugar, and heavy cream in a heavy saucepan. Cook to 239°F (115°C), stirring continuously and scraping the bottom and corners to prevent the dairy from scorching.

Carefully pour the cooked mixture onto the prepared worktable without scraping any crusted residue from the saucepan into the syrup. Place the prepared fondant, melted cocoa mass, and vanilla extract on top of the hot mixture. Do not mix them in yet.

Allow the mixture to cool undisturbed to 120°F (50°C). Use a thermometer rather than estimating the cooling time.

Work the mixture continuously with the plastic scrapers, incorporating the fondant, cocoa mass, and vanilla. The fudge will become thicker, creamier, and increasingly opaque. This takes approximately 8 minutes, but the changing texture determines when to stop.

Transfer the fudge to the prepared frame while it is still soft enough to spread. Do not continue tabling until the mixture becomes completely firm.

Spread the fudge evenly inside the frame. For a smooth top, cover with the second sheet of lightly oiled parchment and level gently with a rolling pin. Alternatively, leave the surface rippled.

Allow the fudge to finish crystallizing before removing the top parchment and cutting, approximately 30 minutes. Wait longer if it is not firm enough to cut cleanly.

Cut into approximately 1-inch squares. Package promptly to prevent the exposed surfaces from drying out.