Line the square tin so the brownies can be handled after cooling, and let the melted butter cool slightly before whisking it with the eggs. Fold the flour only until no dry streaks remain. Look for the center to be set with moist crumbs on the skewer; a clean dry skewer is not the endpoint described by this recipe.
Let the brownies cool completely before dividing into nine. Pipe the crossing lines first, then connect them with curved lines. If a web becomes crowded, stop with a simpler pattern rather than repeatedly covering it in more chocolate. Leave the decorated pieces flat until the chocolate sets, and use a container that will not press its lid onto the raised lines.
Timing and servings are estimates. Equipment, ingredients and portion sizes affect the result.
