Preheat oven to 350 degrees. Spray a 6-cavity doughnut pan and set aside.
Combine flour, salt, baking powder, and lime zest in a bowl. Whisk the ingredients together and set aside.
In a larger bowl, whisk together oil, vanilla, sugar, milk, and egg until well-combined.
Add your dry ingredients and stir until blended. Spoon mixture into pan and bake for 17-18 minutes until the donuts bounce back when touched. Remove from pan and cool on a wire rack.
While your donut cool, whisk the cream cheese, heavy cream, 2 tablespoons powdered sugar, lime juice and vanilla into a wide, shallow bowl. If the frosting become thin, and another tablespoon of powdered sugar.
Dip cooled doughnuts into the frosting and return them to the rack. Repeat with other doughnuts and serve once frosting is set. Garnish with lime zest if you would like!