You'll notice that I call for melted butter for the muffin batter and cold butter for the streusel. This isn't just to keep you guessing, I promise! The melted butter mixes seamlessly into the batter, while the cold butter is essential to create that crave-able crumbly texture for the muffin topping. Try not to overwork (and overheat) the streusel so it stays coarsely textured.