Make the ice cream: Whisk the egg yolks and sugar in a large bowl until pale. Bring the milk, heavy cream, and vanilla pod and seeds to a simmer in a small saucepan. Slowly whisk the hot liquid into the egg mixture; strain through a fine-mesh sieve into a clean bowl, then set the bowl in a larger bowl of ice water. Add the Irish cream and whiskey and stir until cool. Transfer to the refrigerator and chill at least 3 hours. Freeze in an ice cream maker according to the manufacturer's directions.
For each float, put a scoop of the ice cream in a glass. Add the chocolate bitters and top with the Guinness. If you don't have an ice cream maker, fold Baileys Irish Cream and whiskey into softened vanilla ice cream and freeze until firm.
Photograph by Kana Okada