Preheat oven to 350°F and line a baking sheet with parchment paper.
Cream butter and sugars until light and fluffy.
Mix in egg, egg yolk, vanilla, and sourdough discard until smooth.
Whisk flour, baking powder, and salt; add to wet ingredients and mix just until combined.
Gently fold in frozen raspberries and white chocolate.
Scoop 3 tablespoons of dough, place a few inches apart, and bake.
Large cookies: 12–13 minutes
Smaller cookies: 10–11 minutes
Cool slightly, then enjoy.
