Preheat oven to 425 degrees and grease a donut pan with oil spray.
Add flour, sugar, baking powder, cinnamon and sea salt to a large bowl and whisk together until combined.
In a separate bowl combine the buttermilk, eggs, and oil, whisking together with a fork.
Dump the wet ingredients into the dry ingredients. Using a rubber spatula, fold until a batter begins to form and the flour begins to disappear. Be careful not to over-mix as this will make the doughnuts tough and dry. Stop mixing as soon as the last streak of flour disappears and spoon the batter into the piping bag fitted with a large round tip.
Pipe a small amount of batter into each of the 12 greased donut cups in a circular motion and be careful not to over-fill. This batter rises up quite a lot so filling the cups only half-way works perfectly. Sprinkle generously with streusel topping.
Bake for about 7-9 minutes.
Streusel topping from A Dash of Sanity: http://www.adashofsanity.com/2015/06/copycat-starbucks-reduced-fat-cinnamon-swirl-coffee-cake-recipe/
Adapted from The Busy Baker