Preheat oven to 350 degrees F (175 degrees C). Grease 24 muffin cups or line with paper muffin liners.
Heat a large skillet over medium-high heat. Cook and stir grated luncheon meat in hot skillet until browned and crispy, 5 to 7 minutes. Transfer meat to a paper towel-lined plate and cool completely.
Beat cake mix, water, mayonnaise, and eggs together in a large bowl with an electric mixer on low speed until just moistened. Increase speed to medium and beat until smooth, about 2 minutes. Fold 2/3 of the fried luncheon meat into cake batter. Spoon batter into prepared muffin cups.
Bake in the preheated oven until a toothpick inserted into the center of a cupcake comes out clean, about 20 minutes. Cool in the pans set on wire racks until completely cool.
Melt butter in a small saucepan over medium heat. Add brown sugar and heavy cream; stir until sugar dissolves. Bring to a simmer and cook sauce without stirring for 1 minute. Remove from heat, stir salt into sauce, and cool completely.
Transfer sauce to a large bowl and beat in confectioners' sugar 1 cup at a time until frosting reaches desired consistency. Spread frosting onto cooled cupcakes and top with remaining fried luncheon meat.