Preheat the oven to 350°F.
Place the walnuts on a small baking sheet and toast in the oven for 5-7 minutes, or until lightly browned and fragrant. Let cool completely on the pan.
Line a large baking sheet with parchment paper and set aside.
In a heavy saucepan, combine the butter, sugars, whiskey and sea salt. Place over medium-high heat and clip a candy thermometer to the side of the pot. Cook, whisking occasionally, until the mixture reaches 250°F, then whisk constantly until the mixture reaches 298°F. Remove from the heat.
Immediately pour the hot candy onto the parchment-lined baking sheet and sprinkle on the toasted walnuts. Sprinkle over the candy one or two pinches of flaky sea salt.
Let the candy set at room temperature until hard and brittle, about 30 minutes. Break the candy into pieces. Store the candy at room temperature in an air-tight container.