Preheat oven to 350°F and grease donut pans.
Combine flour, sugar, salt, and baking powder to a large bowl.
In a separate bowl, mix melted butter, vanilla, discard, milk, and egg.
Pour wet ingredients into dry and stir together until well combined.
Pour batter into a piping bag or ziplock bag with the corner clipped and pipe batter into donut pan.
Bake for 15-17 minutes.
Remove donuts and cool on a wire rack while you make the glaze.
Make the glaze by adding all of the ingredients to a large bowl and mix together using an electric mixer.
Once the donuts have cooled, dip each one in the maple glaze and let them drip on a wire rack with parchment paper underneath.