When browning butter, watch it closely as it can go from perfectly nutty brown to burnt quickly. Ensure you use unsalted butter for better control over the final seasoning. Adding the balsamic vinegar off the heat prevents it from burning and mellows its acidity slightly while preserving its flavor. Toasting the walnuts enhances their crunch and nutty flavor, providing a lovely textural contrast to the soft ravioli. You can add a pinch of red pepper flakes to the brown butter for a touch of heat.