The Best Marshmallow Brownies for Passover

Name: The Best Marshmallow Brownies for Passover
Ingredients: 8 tablespoons (1 stick) butter or vegan buttery stick, plus more for greasing the pan; 12 ounces semi-sweet or bittersweet chocolate chips (about 2 cups); 2 teaspoons hot tap water; 1 teaspoon instant coffee granules; 2 large eggs; 3/4 cup granulated sugar; 1 teaspoon vanilla extract; 1/2 cup matzo cake meal (see Recipe Note); 1 tablespoon potato starch; 1/4 teaspoon fine sea salt; 1 cup mini marshmallows; 1/2 cup chopped toasted walnuts or pecans (optional)