01 - Preheat oven to 375°F (190°C). Generously grease a standard donut pan with butter or non-stick spray.
02 - In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, and salt until evenly distributed.
03 - In a separate bowl, whisk together the milk, eggs, melted butter, and vanilla extract until smooth and well blended.
04 - Pour the wet mixture into the dry ingredients and fold gently with a spatula until just combined. Do not overmix to keep the donuts tender.
05 - Transfer the batter to a piping bag or use a spoon to fill each donut cavity about two-thirds full.
06 - Bake for 13 to 15 minutes, or until a toothpick inserted into the center of a donut comes out clean. Let cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
07 - In a mixing bowl, beat the softened butter with a hand mixer or stand mixer on medium speed until creamy and pale, about 2 minutes. Gradually add the sifted powdered sugar, beating on low until incorporated, then increase speed and beat until light and fluffy.
08 - Add the strawberry puree, vanilla extract, salt, and pink food coloring if using. Beat until the frosting is smooth, creamy, and easily spreadable.
09 - Once the donuts have cooled completely, spread or pipe the strawberry buttercream frosting generously over the tops. Serve immediately or store in an airtight container.