Line an 8x8 inch square baking pan with parchment paper and set aside.

In a heavy-bottomed 3-quart saucepan, combine the brown sugar, granulated sugar, heavy cream, butter, and salt.

Place the saucepan over medium heat and stir constantly until the sugar dissolves and the mixture begins to boil.

Attach a candy thermometer to the side of the pan, ensuring the bulb does not touch the bottom.

Continue cooking without stirring until the mixture reaches 238°F (114°C), known as the soft-ball stage.

Remove the pan from the heat immediately and allow it to cool undisturbed until the thermometer reads 110°F (43°C).

Add the vanilla extract and optional walnuts to the pan.

Using a wooden spoon or heavy-duty mixer, beat the mixture vigorously until it thickens, loses its glossy sheen, and starts to hold its shape.

Quickly transfer the fudge into the prepared pan and smooth the surface with a spatula.

Allow the fudge to cool at room temperature for at least 1 hour until firm before slicing into 1-inch squares.