Add sugar, water, and corn syrup to a small saucepan over medium-high heat.
Bring to a boil and using a candy thermometer, boil until 275°f or soft crack. Once the temp reaches 250°f, the temp will climb to 275°f fairly quickly.
Pour candy onto a large baking sheet and make sure it covers the whole sheet. Let sit and cool. Let it sit for about 2-3 hours until cool to touch. To break the candy into glass pieces, contort the baking pan. To break it into smaller pieces, use a small hammer or a wooden spoon to tap the candy into small pieces.
Pipe the frosting onto the cupcakes and place 3-4 pieces of candy glass into the frosting.
Apply drops of red food gel to the candy glass to mimic blood. For a splashed effect, water the gel slightly and flick the food coloring onto the frosting and candy glass.
Notes
If you live in a humid climate, candy glass will soften and melt. Serve immediately.