Preheat your oven to 350°F (175°C). Grease a donut pan with non-stick cooking spray or butter, and set aside.
In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until well combined.
In another bowl, whisk together the buttermilk, egg, melted butter, vanilla extract, and fresh strawberry puree until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix; a few lumps are okay.
Transfer the batter into a piping bag or a ziplock bag with the corner cut off. Pipe the batter into the prepared donut pan, filling each cavity about 2/3 full.
Place the donut pan in the preheated oven and bake for 10-12 minutes, or until the donuts are lightly golden and spring back when touched.
While the donuts are baking, prepare the frosting. In a small bowl, whisk together the powdered sugar, fresh strawberry puree, and vanilla extract until smooth. Adjust the consistency by adding more powdered sugar if it's too thin or more strawberry puree if it's too thick.
Once the donuts are baked and cooled for a few minutes, dip the tops of each donut into the strawberry frosting, then place them on a wire rack to set. You can also sprinkle some chopped strawberries on top for extra decoration if desired.