Line a baking sheet, large plate, or cutting board with parchment paper or a silicone baking mat. Set aside.

In a medium heavy-bottomed saucepan, melt the butter over medium heat.

Once the butter is melted, add the light brown sugar, dark brown sugar, and salt.

Whisk continuously until the butter and sugars are fully combined and the mixture looks smooth and glossy, about 1–2 minutes. The mixture may look grainy at first but should become more uniform as the sugars begin to dissolve.

Reduce the heat to medium-low. Continue cooking, stirring occasionally with a heatproof spatula or whisk, until the mixture reaches 295–305°F on a candy thermometer.

The toffee will darken slightly, become bubbly, and develop a rich caramel aroma as it cooks. Watch it closely during the final few minutes because the temperature can rise quickly.

Immediately remove the saucepan from the heat and carefully pour the hot toffee onto the prepared parchment paper or silicone mat.

Use an offset spatula or the back of a spoon to spread it into a thin, even layer if needed.

Allow the toffee to cool completely at room temperature until hard and brittle, about 30 minutes.

Once completely cooled, break or cut the toffee into large shards.

For even smaller toffee bits, transfer the shards to a cutting board and carefully chop them into your desired size using a sharp knife.