Spread butter on the bread slices. Turn over and spread the other side of one slice with goat cheese.
Heat a skillet and place the goat cheese slice butter-side-down in the pan. Layer tomato, arugula, prosciutto and Havarti on top of the goat cheese. Top with a 2nd slice of bread, (butter-side-up).
Cover, and heat the sandwich on low until lightly browned. Carefully flip and toast the other side.
Remove to cutting board. Let sit for a couple of minutes, then cut with a serrated knife and serve immediately.