Prep the pan: Line an 8-inch square pan with parchment or foil and butter the paper so the finished fudge will release easily. Set aside.

Combine dry ingredients: In the heavy saucepan, stir together 3 cups granulated sugar, 2/3 cup dutch-processed cocoa, and 1/8 teaspoon salt so the cocoa is evenly distributed. Use a wooden spoon (not a whisk).

Add the milk and start cooking: Add 1 1/2 cups whole milk to the dry mix and stir until combined. Place the pan over medium heat and stir continuously with the wooden spoon until the mixture comes to a strong boil (a rolling boil). This usually takes about 10–15 minutes depending on your stove.

Cook to temperature: Once you have a strong boil, reduce to low-medium heat and continue cooking without stirring until the candy mixture reaches 234°F (112°C) on a candy thermometer. This stage can take ~30 minutes — be patient and keep the heat steady. (If your thermometer reads in Fahrenheit only, target 234°F.)

Remove and add butter & vanilla: Immediately remove the pot from the heat. Add 1/4 cup butter and 1 teaspoon vanilla to the hot mixture — do not stir after adding. Let the pot sit undisturbed and cool to about 110°F (≈43°C) (room temperature), roughly 30 minutes. This resting step is important for the right crystal structure.

Beat the mixture: After cooling to ~110°F and once the surface has lost some gloss, beat the mixture with a wooden spoon (or heavy-duty mixer on low) until the fudge begins to thicken and loses more shine — about 6–8 minutes of vigorous stirring. You’ll notice it become lighter and start to pull away slightly from the sides. Work quickly but steadily.

Spread into pan: Immediately spread the beaten fudge evenly into the prepared pan using a spatula. Work fast — the mixture will start setting quickly. Smooth the surface.

Cool & cut: Let the fudge cool completely at room temperature (it’s best left overnight) until fully set. Once firm, lift from the pan using the parchment and cut into squares.