Thumbprint cookies are a five-ingredient shortbread dough rolled in granulated sugar, pressed with an indentation, and filled with jam or chocolate. They bake in under 15 minutes and stay moist for days.
Preheat the oven to 325°F conventional (no fan) and line one baking sheet with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, combine the butter, powdered sugar, flour, salt, and vanilla extract. Mix on medium-low speed until a smooth dough forms.
Pinch off pieces of dough about an inch across and roll into balls. Roll each ball in granulated sugar. You will have approximately 15 cookies.
Place the balls on the prepared baking sheet about an inch apart. Press your thumb or forefinger gently into the center of each cookie to create an indentation that goes about halfway through.
Fill the indentations with jam before baking. If using a chocolate filling, bake the cookies unfilled and add the filling after they cool.
Bake for 12 to 15 minutes, rotating the tray after 6 minutes, until the surface looks matte and no longer glossy.
Cool completely on a wire rack before serving or storing.
Notes
Yield: 15 cookies, approximately 1 inch in diameter each. Doneness Cue: The surface will shift from shiny to matte. Pull the cookies as soon as the gloss disappears, starting to check at 12 minutes. Storage: Store in an airtight container at room temperature for up to 10 days. Cookies taste best within the first 5 days, though jam-filled cookies stay soft and moist for the full 10 days.