Line an 8-inch square baking pan or a 2-quart baking dish with foil. Coat foil with cooking spray; dab away the excess with a paper towel.Advertisement
In a large, heatproof bowl, combine chocolate and butter. Set bowl over a pan of simmering water over medium-low heat, making sure water doesn’t touch bowl. Stir until completely melted. Remove bowl from heat. Add sweetened condensed milk, fine sea salt and vanilla; stir to blend. Stir in flaky sea salt and nuts. Spread in prepared pan, smoothing the top.
Refrigerate until set, about 1 hour. Remove fudge from pan; cut into 1-inch squares. Store in an airtight container in the refrigerator up to 1 month.
Immediately after toasting nuts, lightly sprinkle with fine sea salt.