Step 1Grind 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 2In 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.