Line a 13x9 inch metal baking pan with foil, leaving overhang.

Combine chopped bittersweet chocolate, unsalted butter, chopped unsweetened chocolate, and instant espresso powder in a large bowl.

In a heavy, large, deep saucepan, stir together granulated sugar and fat-free evaporated milk over medium heat.

Continue stirring until the sugar dissolves and the mixture comes to a boil.

Attach a clip-on candy thermometer to the side of the pan.

Boil the mixture until the thermometer registers 234°F (soft-ball stage), stirring constantly.

Adjust the heat as needed to avoid boiling over, which should take about 10 minutes.

Immediately pour the hot milk mixture over the chocolate mixture in the bowl.

Whisk until the chocolate and butter melt and the fudge is smooth and well combined.

Mix in the toasted walnut pieces and vanilla extract.

Spread the fudge evenly into the prepared baking pan.

Refrigerate the fudge uncovered until it is cold and set, which should take approximately 3 hours.

Once set, use the foil overhang as an aid to lift the fudge from the pan.

Cut the fudge crosswise into 7 strips.

Then, cut each strip lengthwise into 5 strips, resulting in 35 pieces.

Return the cut fudge to the pan, cover, and chill until ready to serve.

The fudge can be made up to 2 weeks in advance.

Keep the fudge covered and chilled until serving.

Before serving, bring the fudge to room temperature for optimal texture and flavor.