Preheat oven to 400°F (200°C). Butter and flour 2 (6-well) doughnut pans.
In a small bowl, whisk together flour, salt, baking powder, and baking soda.
In a medium bowl, whisk together granulated sugar and lime zest until zest is evenly distributed. Add buttermilk, egg, 3 tablespoons lime juice, and oil. Whisk until combined. Add flour mixture, and whisk until just combined.
Transfer batter to a pastry bag, and cut a ½-inch opening in tip. Pipe batter evenly into prepared wells.
Bake until puffed and edges are golden, about 8 to 10 minutes.
Let cool in pans for 10 minutes. Invert doughnuts onto a wire rack, and let cool completely.
In a small bowl, whisk together confectioners’ sugar and remaining lime juice. Tint with food coloring as desired. Carefully dip top of each doughnut into glaze.
Garnish with graham cracker crumbs and lime zest, if desired. Store in an airtight container for up to 3 days.
