Step 1

Line an 8-inch square baking pan with aluminum foil.

Step 2

Place chocolate in a food processor; pulse until finely chopped.

Step 3

Pour evaporated milk into a microwave-safe bowl. Stir in sweetener, 1/2 cup at a time, until dissolved. Add vanilla extract. Stir in chopped chocolate.

Step 4

Heat mixture in the microwave in 30-second intervals, stirring after each interval, until chocolate is melted and smooth, about 2 minutes total. Stir in walnuts.

Step 5

Spread chocolate mixture in the prepared pan. Refrigerate until firm, about 2 hours. Cut into 1 1/2-inch squares.