Preheat the oven to 325 degrees F (165 degrees C).
Mix butter and powdered sugar together with a rubber spatula until creamy. Add vanilla extract, almond extract, salt, and egg yolk. Mix with the spatula. Add flour and blend until just combined.
Use a sorbet scoop to portion dough into 1/2-ounce (1-tablespoon) balls. Roll balls in a plate of white sugar to coat; roll again between your palms. Place several inches apart on a silicone-lined baking sheet. Flatten balls lightly with your fingers.
To make shortbread cookies with a little jam, poke a well into each cookie using the end of a thick wooden spoon dusted with powdered sugar or using one of your fingers. Fill cookies with fruit jam. Tap baking sheet against the counter to let jam settle.
Bake in the preheated oven until tops are barely blonde and bottoms are slightly golden, about 15 minutes. Let cool on a wire rack before icing.
Meanwhile, mix powdered sugar with a little milk until you have a consistency that will hold its shape when piped. Pipe icing on top of cookies in a zigzag pattern. Let sit for 15 minutes before serving.
For a more jam-forward cookie, press your thumb into the cookies to make large, shallow indentations. Spoon in jam. Tap baking sheet against the counter.
Bake until not quite golden brown, about 15 minutes. Let rest on pan for 5 minutes before transferring a wire rack. Dust with powdered sugar.
For best results, do not chill the dough.
For the icing, feel free to use water instead of milk.
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