In a medium heatproof bowl, whisk yolks, sugar, and flour for 1-2 minutes or until yolks lighten in color. If the mixture is too thick to whisk, add a small amount of the heavy cream.
Heat the cream in a medium saucepan and bring it to a simmer. Slowly add the cream to the yolk mixture, whisking constantly until well blended.
Pour the mixture from the bowl back into the same saucepan. Heat the mixture over medium-high heat, whisking constantly until custard thickens slightly and coats the back of a spoon – about 3-4 minutes. Do not let it come to a boil.
Immediately pour the custard through a sieve set over a bowl set into a larger bowl of ice water.
Remove the sieve and whisk the buttermilk and vanilla gently into the cooling custard.
Allow the custard to cool to room temperature. Cover the bowl with plastic wrap and chill overnight.
Process the custard in an ice cream machine, and store in an airtight container in the freezer.