Step 1 Grind the almonds in a blender or food processor, then place them in a saucepan with the half-and-half. Heat to just under a boil, turn off the heat and let sit for 30 minutes. Strain the mixture through a very fine strainer or cheesecloth. Return the mixture to a clean saucepan. If using extract, add it to the custard after it's been cooked with the eggs.
Step 2 In the bowl of an electric mixer, beat the sugar and egg yolks together until thick and lemon-colored. With a whisk, mix the yolk and sugar mixture into the half-and-half. Cook over low heat, stirring constantly, until custard coats a spoon, 10 to 15 minutes. Chill for 1 hour, add the almond extract and freeze in an ice cream maker according to manufacturer's instructions.