Preheat oven to 350°F. Spray a donut pan with baking spray.
Whisk flour, baking powder, baking soda, and salt in a bowl. Set aside.
1 cup all-purpose flour, ½ teaspoon baking powder, ½ teaspoon baking soda, ½ teaspoon salt
Whisk egg, sugar, oil, sour cream, and vanilla extract in a large bowl. Add dry ingredients and stir by hand just until combined.
1 large egg, ½ cup granulated sugar, ¼ cup vegetable oil, ¼ cup sour cream, 1 teaspoon vanilla extract
Stir in chopped strawberries. Transfer to a piping bag and snip off the end. Pipe enough batter into each donut cavity to fill them about 2/3 full.
½ cup fresh strawberries
Bake for 13-15 minutes until golden brown on top. Cool for 5 minutes, then transfer to a wire rack to cool completely.
Make the glaze by whisking powdered sugar and mashed strawberries together in a medium bowl. It should fall back into the bowl in a thick ribbon.
1 cup powdered sugar, 2 Tablespoons fresh strawberries
The glaze should slowly drip off the end of the spoon in a thick ribbon. Dip each doughnut about halfway into the glaze. Allow the excess to drip off. Place the doughnuts back on the cooling rack and let them set for 10 minutes.
Dip the donuts into the glaze to cover the tops, then let the excess glaze drip off back into the bowl. Return the glazed strawberry donuts to the wire rack until the glaze has set up, about 15-30 minutes.