1. Adjust rack to lower third of oven and preheat oven to 325 degrees F. Press a sheet of aluminum foil to cover outside the bottom and sides of a 9-inch square pan. Invert pan and gently press aluminum form into pan to fit contours; set aside.

2. Crust: In a large bowl, blend the graham cracker crumbs and the sugar. Add the butter and blend thoroughly. Press the crumbs into foil-lined pan; set aside.

3. Filling: Sift the flour, salt, and baking soda onto a piece of waxed paper; set aside. In a small saucepan, melt the chocolate and butter over low heat, stirring occasionally. Turn off heat, stir in the water. Pour the mixture into a large bowl and let cool for about 5 minutes. Stir in the sugar, then the eggs and vanilla, just until thoroughly blended. Add the flour mixture, stirring just until combined. Pour filling into crumb-lined pan, spreading evenly.

4. Bake for 30 minutes only. Remove pan from oven to a rack, and cool in the pan.

5. Topping: To serve, remove the cake from the pan to a cutting board by lifting the foil by its edges. Pour the chocolate into a small handmade paper cone and pipe zigzag lines over the filling’s surface. Cut into 1½-inch squares. Center a mini marshmallow on each square, and pipe a tiny dot of chocolate in the center of each marshmallow.

6. Store in one layer in a covered foil-lined cardboard container, such as a cake box, at room temperature up to 2 days.