Thoroughly wash the grapefruit under warm water to remove any wax or residue.
Cut the grapefruits in half and juice them, reserving the juice for other uses.
Using a spoon, gently scrape away most of the white pith from the inside of the peels, leaving about 1/4 inch for structure.
Cut the peels into strips about 1/4 inch wide.
Place the peel strips in a large saucepan and cover with cold water. Bring to a boil, then drain immediately.
Repeat this blanching process two more times with fresh cold water each time.
In the same saucepan, combine 4 cups water and 2 cups sugar. Bring to a simmer, stirring until the sugar is completely dissolved. Add the corn syrup and stir to incorporate.
Add the blanched peel strips to the sugar syrup and simmer gently for 45-60 minutes, until the peels become translucent and the syrup thickens.
Remove from heat and stir in vanilla extract if using. Allow the peels to cool in the syrup for about 15 minutes.
Using a slotted spoon, carefully transfer the peels to a wire rack positioned over parchment paper. Separate them to ensure they don't stick together.
Allow the peels to dry for 2-3 hours until they're still slightly tacky but not wet.
Roll the slightly tacky peels in granulated or superfine sugar until completely coated.
Return the sugar-coated peels to the wire rack and allow them to dry completely for at least 24 hours.
Store in an airtight container with parchment paper between layers to prevent sticking.