In a small bowl, combine mascarpone, dulce de leche and vanilla extract. Butter one side of each slice of bread. Spread mascarpone mixture on non-buttered side of 4 slices of bread. Spread raspberry preserves on non-buttered side of remaining 4 slices of bread. Place one slice of bread with raspberry preserves on each mascarpone-topped bread slice, buttered sides out.
Place sandwiches on a preheated 12-inch stovetop griddle or skillet. Grill 1-2 minutes per side over medium high heat, or until bread is lightly toasted. Remove and serve immediately.