Step 1

Line a baking sheet with parchment paper. Combine butter, sugars, whiskey, and sea salt in a heavy saucepan. Cook over medium-high heat, whisking occasionally, until mixture reaches 250ºF. Whisk constantly until mixture reaches 300ºF, then immediately pour mixture out on prepared baking sheet and spread evenly with an offset spatula. Place baking sheet in the freezer for 10 minutes to set toffee.

Step 2

Place chocolate in a double boiler or in a heat-proof bowl over a saucepan of gently simmering water. Slowly melt the chocolate, stirring occasionally until glossy and smooth. Pour melted chocolate over cooled toffee layer and spread with an offset spatula into an even layer; sprinkle with chopped almonds and pecans. Let chocolate set completely, then break toffee into pieces. Store leftovers in an airtight container at room temperature.