For best results, use high-quality spices for a more pronounced flavor. Do not overmix the batter, as this can result in tough muffins. The streusel topping can be made ahead of time and stored in the refrigerator for up to 2 days. Feel free to substitute pecans or almonds for the walnuts in the streusel or the batter. These muffins can be stored in an airtight container at room temperature for up to 3 days.