Chocolate Walnut Marshmallow Fudge

Grease an 8-inch square pan with cooking spray and line with parchment paper.

Place chocolate in the top of a double boiler over simmering water. Stir constantly with a rubber spatula, scraping down the sides with a rubber spatula to avoid scorching, until chocolate melts, about 5 minutes.

Stir in condensed milk, vanilla extract, and salt; the chocolate may seize up temporarily. Continue stirring over medium to medium-high heat until mixture is smooth, about 2 minutes. Stir for 1 more minute, then remove from the heat.

Add marshmallows and walnuts. Stir with a wooden spoon until fully incorporated; the marshmallows will not melt. Transfer to the prepared pan. Cover with plastic wrap and flatten with your hands. Remove plastic wrap and sprinkle with sugar and sea salt.

Place in the refrigerator until hard, about 2 hours. Remove from the refrigerator and cut into 36 pieces.