Preheat oven to 375 degrees.
Spray donut pan with non-stick cooking spray.
In a large bowl, whisk together flour, baking powder, baking soda and salt.
In a small bowl mix together egg, brown sugar, buttermilk, vegetable oil, lime zest and key lime juice. Add to dry ingredients and mix just until moistened.
Spoon batter into prepared donut pan.
Bake 9 minutes or until cooked through.
Remove from oven and let cool for 10 minutes before removing from pan.
For glaze: In medium bowl whisk powdered sugar, key lime juice, milk and food coloring until smooth. Add more milk, if needed, to get to desired consistency. Invert donut to glaze, letting excess drip off. Place on wire rack and top with graham cracker crumbs. Serve.