Step 1In a large, heavy saucepan, combine the cream, milk, corn syrup, sugar and salt over medium-high heat. Cook, stirring occasionally, until the mixture comes to a boil. If any sugar crystals begin to form on the edges of the pan, wash down the sides with a wet pastry brush.
Step 2Insert a candy thermometer into the mixture and continue cooking until the syrup reaches 243 degrees. Remove from heat and pour into a clean 13-by-9-inch baking dish. Cool to room temperature.
Step 3Add the vanilla seeds and invertase. Begin stirring the fondant with a bench scraper or wooden spoon, working the fondant until the ingredients are completely incorporated. Move the dish, uncovered, to the refrigerator until the fondant is cold. Work the fondant again (it will be thickened and somewhat stiff), stirring and then kneading until it is smooth and has lost its sheen.
Step 4Separate 1/3 of the mixture; this will become the yellow centers of the eggs. Add a few drops of yellow food coloring and knead the coloring into the fondant, dusting your hands and work surface with powdered sugar to keep the fondant from sticking. Form the fondant into a thin, flat disk; cover tightly with plastic wrap and freeze until the fondant is firm, at least 2 hours. Meanwhile, form the remaining white fondant into a thin flat disk and freeze until very firm.
Step 5Form the “yolks”: Measure 1 teaspoon of the yellow fondant and roll it in a ball (if the fondant is sticky, flour your hands with powdered sugar). Repeat with the rest of the fondant, placing the yellow balls on a parchment-lined rimmed baking sheet lightly dusted with powdered sugar. Work quickly, as the fondant will become very sticky as it warms up; if necessary, freeze the fondant until firm before continuing. Freeze the balls, uncovered, until firm, 30 minutes to an hour.
Step 6Meanwhile, measure 2 teaspoons of the remaining white fondant to form each of the “egg whites.” When the yellow balls are firm, form the whites around each of the balls, molding the white fondant into an egg shape. Transfer the eggs to a parchment-lined rimmed baking sheet lightly dusted with powdered sugar. Gently insert a wooden skewer through the center of the egg, at the larger end, and push the toothpick through just over half the length of each egg. Freeze the eggs, uncovered, until very firm, at least 2 to 3 hours, preferably overnight.
Step 7Melt the chocolate in a bowl set over simmering water. Transfer the melted chocolate to a small deep bowl (this will make it easier to coat the eggs). Working 1 to 2 eggs at a time (keep the rest frozen until ready to coat), quickly dip the eggs in the chocolate to coat, gently tapping the skewer on the rim of the bowl to shake off excess chocolate. Press the toothpicks onto a piece of foam to allow the chocolate to settle and harden. The chocolate should harden quickly; if it takes a while to set, place the eggs back in the freezer just until the chocolate is hard.
Step 8Remove and discard the toothpicks from each of the eggs, then add a small dollop of chocolate to seal the holes. Place the eggs aside in a cool, dry place to set completely. The eggs can be eaten right away; the invertase should soften or liquify the filling within 3 to 5 days at room temperature.