Melt butter in a small saute pan over medium heat and add the sugar. Stir until sugar is melted and remove from heat. Stir in the pecans and transfer the mixture to a baking sheet. Bake for about 5 minutes until toasted. Lay the prosciutto slices on another baking sheet and bake for 2 to 3 minutes until the prosciutto is crispy. In a large salad bowl, place the spinach, candied pecans, cranberries, and prosciutto. Toss with the dressing and garnish with crumbled feta.