Preheat oven to 350 º F (177 º C).
In a medium mixing bowl, whisk together the flour, baking powder, sea salt, and cinnamon. In a small separate bowl, whisk the egg. Set both aside.
In a small saucepan over medium-low heat, combine the apple cider, sugar and butter, stirring continuously until butter has just melted. Remove from heat, let cool for 2 minutes, and stir in the apple cider vinegar, vanilla, and whisked egg. Slowly pour the wet ingredients into the dry; stir well with a whisker until combined and almost no lumps are visible.
Using a tablespoon or a cookie scooper, spoon/scoop batter into a donut pan (3/4 full). Bake in the oven for 12-14 minutes (standard-size, yielding about 6 donuts) or for 6-7 minutes (mini baked donuts, yielding about 26), or until light brown and cooked through. Let cool for 2-3 minutes on a rack. Then, remove baked donuts from pan and place them directly on a rack to cool completely before dipping into glaze.
For the Maple Glaze: Whisk everything together in a small bowl until smooth. Set rack of donuts over a baking sheet lined with parchment paper. Dip the top of the baked donuts into the maple glaze, turning to coat well. Let baked donuts rest on rack until glaze sets.