In a small saucepan over high heat, dissolve the sugar in half of the water (1/4 cup/60 ml). Cook without stirring until the caramel becomes light amber in colour. Remove from the heat. Add the remaining water (1/4 cup/60 ml) and the corn syrup. Beware of splattering. Return the saucepan to the heat and let simmer until a candy thermometer reads 242°F (117°C).
Beat the egg yolks with an electric mixer at medium speed. Drizzle the hot caramel onto the yolks, avoiding the mixer’s beaters. Beat continuously at high speed until completely cooled.
Add the butter, 1 or 2 pieces at a time, beating until smooth and creamy. With a spatula, scrape the bottom and sides of the bowl several times during this step.
Can be made in advance, just bring to room temperature and beat before using.