Generously dust powdered sugar over your work area. Fill your sink or a large bowl with cold water.
Place the sugar and water in a large, heavy saucepan and heat gently, stirring until the sugar dissolves.
Add the cream of tartar and bring to a boil. Cover and let the sugar continue to boil for 3 minutes.
Uncover and boil until the temperature reaches 240 °F on a candy thermometer. This is also known as the soft-ball stage.
Carefully place the bottom of the saucepan in the cold water, stirring until the mixture becomes thick and creamy.
Stir in almonds, egg whites, and almond extract.
Return the pan to the stove over low heat. Stir until thickened, about 2 minutes.
Spoon the mixture onto your prepared work area. Turn the marzipan with a metal spatula until it's cool enough to touch.
Dust your hands with powdered sugar and knead the marzipan until smooth and pliant.
Use immediately or wrap in plastic wrap and store in an airtight container. It will keep up to three months in the fridge or six months in the freezer. Bring to room temperature before using.