Combine the egg, flour, and ½cup sugar in the bowl of an electric mixer. Start beating the mixture until it is thoroughly blended, then pour in the hot milk. Pour and scrape the sauce into a saucepan and start beating over moderate heat. Cook until the mixture is thickened and smooth.
Put the remaining ½cup sugar in a small nonstick skillet and cook, stirring, until the sugar is melted and starts to take on a light caramel color. Immediately add the orange rind and blend, stirring constantly. Remove from the heat and add the caramel-orange mixture to the sauce. Blend well.
Line a mixing bowl with a sieve and pour the sauce into it, pressing all around with the sides of a rubber spatula to extract as much flavor as possible from the orange rind. Discard the rind.
Add 1cup heavy cream and stir. Let cool. Pour the mixture into the container of a hand-cranked or electric ice cream maker and freeze according to the manufacturer’s instructions.
Meanwhile, make the sauce. Put the sugar in a 12-inch nonstick skillet and cook, stirring, until the sugar is melted and becomes dark caramel in color. Add the corn syrup, stir to blend, and remove from the heat. Stir in the butter to blend. Stir in the ⅔cup cream and the bourbon. Serve with the ice cream.