Beat softened butter and sugar together until light and fluffy, about 2-3 minutes on medium speed.

Beat in the egg yolk, vanilla extract, and sourdough discard until well combined. Mixture may look slightly curdled—that's fine.

In a separate bowl, whisk together the flour, baking powder, and salt.

Add the dry ingredients to the wet ingredients. Fold with a spatula until just combined. Don't overmix or the cookies will be tough.

Place plastic wrap or a lid directly on the dough surface.

Chill for 30 minutes.

While dough chills, preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.

Scoop dough and roll into balls about 2.5cm (1 inch) in diameter.

Place balls 5cm (2 inches) apart on lined baking sheets.

Use your thumb or the back of a small spoon to press a well into the center of each cookie. If the edges crack, gently press them back together.

Spoon about ½ teaspoon of jam or jelly into each indent. Don't overfill—the jam bubbles and expands during baking.

Roll dough into 2.5cm (1 inch) balls for standard size, or 4cm (1.5 inch) for larger cookies.

Press thumb or spoon back into center to create a well for the filling.

Add ½ teaspoon jam per cookie. For larger cookies, use up to 1 teaspoon.

Preheat oven to 350°F (175°C) with rack in center position.

Bake for 12-15 minutes, until edges are just starting to turn golden. Centers will look slightly underset—that's what you want.

Let cookies cool on the baking sheet for 5 minutes. They'll firm up as they cool.

Move to a wire rack to cool completely before storing.