Heat oven to 350 F.
In large bowl, whisk flour, sugar, baking powder and salt.
Add buttermilk, eggs, butter, lemon juice, vanilla extract and lemon zest. Stir until combined.
Pipe batter into donut pan.
Bake 9 minutes. Cool on wire rack.
To make topping: In bowl, combine sugar with lemon zest. In separate bowl, place melted butter.
Place donut face down in melted butter then face down in lemon sugar topping. Repeat with all donuts.