In a large bowl (or bowl of a stand mixer with paddle attachment on low speed), cream unsalted butter and white sugar until smooth. Scrape the sides of the bowl.
225 g unsalted butter, 150 g White sugar
Add the egg, sourdough starter discard, and vanilla extract to the butter mixture. Mix on medium speed until uniform.
90 g sourdough discard, 1 large egg, 5 g vanilla extract
In a small bowl (or medium bowl), whisk all-purpose flour, salt, and baking powder.
400 g all-purpose flour, 6 g salt, 2 g baking powder
Add dry to wet; mix until no streaks. Stop early.
Wrap in plastic wrap; chill 3 hr min. This dough rest gives best flavor.
Roll dough to 6 mm with a rolling pin. Cut with favorite cookie cutters — a 3-inch cookie cutter is great for Easter. Move unbaked cookies to a lined baking sheet.
Slide the prepared baking sheet into the freezer 10 min for crisp edges.
Bake at 350°F for 8–10 min on a parchment paper-lined cookie sheet until edges are barely golden.
5 min on pan, then a wire rack or cooling rack.
Notes
The 10-minute freeze before baking is the single most important step for the perfect chewy texture.