Step 1

Grease an 8 x 8-inch baking pan with butter (or margarine) and set aside.

Step 2

Preheat the oven to 350°F.

Step 3

Combine the chocolate chips and butter (or margarine) in a microwave-safe bowl and microwave on high until the butter and chocolate are melted and smooth when stirred, 60 to 90 seconds total; set aside to cool slightly.

Step 4

Whisk the eggs, sugar, vanilla, and coffee in a medium bowl and set aside.

Step 5

In another bowl whisk together the cake meal, potato starch, and salt.

Step 6

Whisk the egg mixture into the dry mixture until incorporated, then fold in the melted chocolate mixture until incorporated. Fold in the marshmallows and nuts until just combined.

Step 7

Pour the batter into the prepared pan and bake until the top is slightly cracked and shiny but the center is still slightly soft, 35 to 40 minutes. Cool.