Line an 8x8 inch pan with parchment paper, letting it overhang the sides.

In a heavy-bottomed saucepan, whisk together 2 cups granulated sugar and 3/4 cup unsweetened cocoa powder until well combined.

Add 1/2 cup whole milk to the saucepan and stir constantly over medium heat until smooth and lump-free, about 1–2 minutes.

Stir in 1/4 cup unsalted butter and continue stirring until the butter is melted and the mixture is smooth, about 2 minutes.

Stop stirring and let the mixture cook undisturbed until it reaches 236–238°F (soft-ball stage) on a candy thermometer, about 5–8 minutes. Or use the cold water test.

Remove the saucepan from heat immediately and set it on a cool surface.

Let the mixture cool at room temperature for about 30 minutes, until warm but not hot to the touch.

Stir in 1 teaspoon vanilla extract and a pinch of sea salt until evenly distributed.

Pour the fudge into the prepared pan and spread evenly with a spatula. Let it set at room temperature for about 1 hour.

Lift the set fudge using the parchment overhang and cut into 16 squares with a sharp, warm knife, wiping the blade between cuts.