In a medium bowl, combine 1 cup (120 g) all-purpose flour, ½ cup (50 g) cocoa powder, 1 teaspoon baking powder, ½ teaspoon salt, and whisk together.
In a large mixing bowl, combine ¼ cup (50 g) sourdough discard, ¼ cup (45 g) avocado oil, ½ cup (100 g) light or dark brown sugar, ½ cup (100 g) granulated sugar, 1 egg, 1 teaspoon vanilla extract, and whisk until smooth.
Add the dry ingredients to the wet ingredients and stir together until fully combined.
Cover the dough tightly and refrigerate it for at least 3 hours or up to 3 days.
Preheat the oven to 350°F (177°C). Line two baking sheets with parchment paper or silicone baking mats.
Add ¼ cup powdered sugar to a large plate.
Using a medium cookie scoop, scoop and roll the dough into balls using your hands (about 1.5-2 tablespoons of dough or about 33-35 grams each). Roll each ball very generously in the powdered sugar.
Place 2 inches apart on the baking sheets.
Bake the cookies for 9-11 minutes. Let them cool slightly on the baking sheet for 15 minutes before transferring them to a wire rack to cool completely.