Just like the ones sold in those iconic blue tins but Martha's are even tastier! The secret is using the best-quality salted butter you can find.
Preheat oven to 325 degrees.
In the bowl of an electric mixer fitted with the paddle attachment, beat together butter and confectioners' sugar on medium speed until light and fluffy, about 3 minutes. Add vanilla and egg, and beat to combine. Gradually add flour, and beat until well incorporated. Transfer dough to a pastry bag fitted with a 7/16-inch star tip (such as Ateco #825).
Pipe 2 1/2-inch rings onto parchment-lined baking sheets, spaced 2 inches apart. Bake, rotating halfway through, until lightly golden around edges but still light on top, about 20 minutes. Transfer sheets to wire racks; let cool completely.