Preheat oven to 400°F. Spray a donut tin with nonstick baking spray. Set aside.
In a medium bowl, whisk the flour, baking powder, and salt together. Set aside.
1½ cups all-purpose flour, 1 teaspoon baking powder, ¼ teaspoon kosher salt
In a large bowl, beat the egg and sugar together with a whisk until smooth. Add the oil, buttermilk, and vanilla. Whisk until combined.
1 large egg, ½ cup granulated sugar, ½ cup buttermilk, ¼ cup vegetable oil, 1 teaspoon pure vanilla extract
Add the dry ingredients to the wet ingredients. Mix until just combined.
Gently fold in the chopped strawberries.
½ cup chopped fresh strawberries
Pour the batter into a piping bag or Ziplock bag. Cut the end off of the piping bag or corner of the Ziplock bag and fill the donut cavities ¾ full.
Bake for 8-10 minutes, or until a toothpick inserted into the donuts comes out clean.
Allow the donuts to cool for 10 minutes in the tin. Flip the donut tin upside-down over a wire rack to completely cool.
Make the glaze. Place the finely diced strawberries into a medium bowl. Use a fork to mash and puree the strawberries until almost smooth. Add the vanilla, milk, and powdered sugar and mix until combined. If the glaze is too thin, add more powdered sugar. If the glaze is too thick, add milk.
2 strawberries, ½ teaspoon pure vanilla extract, 2 cups powdered sugar, 1-2 tablespoons milk
Dip the donuts into the glaze or drizzle the glaze over the donuts.