Spread softened butter evenly over both sides of each bread slice.
Cook bread slices in hot skillet until browned, 2 to 3 minutes per side. Cut grilled bread diagonally into 2 triangles.
Melt 1/4 cup butter in a skillet over medium heat; cook and stir chicken livers, prosciutto, sage, salt, and pepper in melted butter until the livers begin to firm, about 5 minutes.
Remove chicken liver mixture from the skillet.
Top each bread triangle with a portion of the chicken liver mixture.
Pour Marsala wine into the skillet and bring to a boil while scraping the browned bits of food from the bottom of the skillet with a wooden spoon; simmer 3 minutes.
Cook and stir 1 tablespoon butter in the Marsala wine mixture until melted; drizzle over the chicken liver topping.
