Using a 2 1⁄2-inch heart-shaped cutter, cut 12 shapes from frozen bread slices, discarding scraps.
Using the same heart-shaped cutter, cut 18 shapes from prosciutto, 6 from Gouda cheese, and 6 from Muenster cheese, discarding scraps.
Spread a thin layer of mayonnaise onto both sides of each bread heart. Top 6 bread hearts with 1 Gouda cheese heart, 3 prosciutto hearts, and 1 Muenster cheese heart. Top with remaining bread hearts to make a total of 6 sandwiches.
Heat an 8-inch skillet over medium-low heat. Working with 2 sandwiches at a time, carefully place sandwiches in skillet and cover with lid; toast sandwiches until golden brown on both sides, approximately 1 minute per side. Using a spatula, carefully remove sandwiches from skillet and onto a serving plate, and let cool for 5 minutes. Serve warm.
Recipe by TeaTime Magazine at https://teatimemagazine.com/heart-tea-grilled-cheese-prosciutto-sandwiches/