1) Line an 8" x 8" square or 9" round pan with parchment, and grease the parchment. If you don't have parchment, use waxed paper or plastic wrap.
2) Combine the chocolate, cream, espresso powder, corn syrup, and vanilla in a microwave-safe bowl.
3) Heat the mixture in the microwave (or in a saucepan set over low heat on the stove) till the cream is hot and the chocolate has softened.
4) Remove from the heat, and stir until the chocolate has melted and everything is smooth.
5) Spread about 3/4 cup of the melted chocolate in the bottom of the pan.
6) Space the marshmallow halves atop the chocolate. Note: feel free to substitute mini marshmallows for the marshmallow halves here. Distribute the walnuts evenly in the spaces between the marshmallows.
7) Spread the remaining chocolate atop the marshmallows and walnuts, spreading to the edge of the pan so that everything is covered.
8) Refrigerate for an hour or so, to set the chocolate. Remove from the refrigerator, and loosen the candy from the edges of the pan.
9) Bring to room temperature, and leave in the pan, covered with plastic, till you're ready to cut into squares.
Hi Jim- I don't think you would want to use the marshmallow fluff as it will most likely be too soft to cut into squares, and would subsequently melt into a puddle if you did manage to get it cut. If you have any other questions, please do give our Baker's Hotline a call at 855-371-2253. Happy Baking! Jocelyn@KAF