In a medium/large bowl, whisk together flour, sugar, salt, and baking powder.
In a small/medium bowl, lightly whisk the egg. Mix in the kefir, vegetable oil, and vanilla extract.
Fold the wet ingredients into the dry ingredients just until the flour streaks disappear. Then, fold in the crushed Oreos.
Pipe or spoon donut batter into the greased donut pan (each one should be about ¾ full). Bake until the donuts spring back from a gentle poke (20-24 minutes). Remove from pan and transfer to wire rack once cool enough to touch.
Chocolate Ganache
Place chocolate chunks evenly into the bottom of a small/medium bowl. Heat the heavy whipping cream in the microwave or stove until it's simmering. Pour hot cream over chocolate chunks, then cover the bowl for 5 minutes.
After 5 minutes, remove the cover and stir the mixture in a circular motion with a rubber spatula until it's a smooth chocolate ganache.
Dip cooled donut tops into the ganache and sprinkle with extra crushed Oreos if desired.
Notes
Donut Pan: Donut quantity and bake time may vary based on donut pan size. This recipe is written for a deep donut pan. If using a shallow donut pan, it may yield more donuts and bake for less time.Storage: Best enjoyed same day. Store unglazed donuts at room temperature for 3 days or in the freezer for 1 month. Glazed donuts can be stored in the refrigerator for 2-3 days.Nutrition: The provided nutrition information is an estimate per donut, assuming 6 donuts are made. The calorie accuracy is not guaranteed.