To get that high dome top, muffins need to initially be baked at a high temperature for 5 minutes and then a slightly lower temperature for the rest of the bake time. That first blast of high heat will force the muffins to bake upward more rapidly, creating a domed top. Then, the temperature is lowered for the remaining 15-18 minutes to bake the muffins all the way through. Spacing them out in the muffin pan will also help, as the heat can be distributed more evenly around the muffins.