If you haven't mixed it up yet, just use less liquid. If you've mixed it and are trying to fix it, just make half of a cake donut recipe and use less liquid than the recipe calls for, then mix into your batter. Use a lot less liquid at first, you can add more at the end if needed.

Or just mix up the dry ingredients in the recipe and add as much as needed to your too runny batter to get the right consistency.