Preheat oven to 175°C (350°F). Line a baking sheet with parchment paper.
Whisk together flour, baking powder, and salt in a medium bowl. Set aside.
250 g all-purpose flour, 5 g baking powder, 2 g salt
In a large bowl, cream butter and sugar until light and fluffy.
115 g unsalted butter, 120 g granulated sugar
Add egg, vanilla, and sourdough starter. Mix until smooth.
1 large egg, 5 ml vanilla extract, 60 g active sourdough starter
Gradually add dry ingredients and mix until soft dough forms.
Scoop dough into 20–25 g portions, roll into balls, and roll in sugar if desired. Flatten slightly and place on the baking sheet.
Bake for 10–12 minutes, until edges are lightly golden.
Cool on baking sheet 5 minutes, then transfer to a wire rack.