01 - Preheat the oven to 400°F.
02 - Trim strawberries and pulse in a food processor to achieve a chunky puree without liquefying. Set aside.
03 - In a large mixing bowl, cream together butter, sugar, and egg on medium-high speed for about 2 minutes.
04 - Incorporate vanilla extract, buttermilk, and strawberry puree into the wet mixture. Mix until fully blended.
05 - Add cake flour, all-purpose flour, baking soda, baking powder, and salt to the bowl. Mix on low speed until just combined. Scrape down the sides as necessary. Do not overmix.
06 - Transfer the batter into a large piping or freezer bag using a rubber spatula. Fold the bag over a large cup for stability, then close and snip off the corner.
07 - Generously grease donut molds. Pipe the batter into each cavity, filling only halfway.
08 - Bake on the middle oven rack for 7 to 8 minutes, or until set. Do not overbake.
09 - While baking, whisk powdered sugar and milk in a bowl until smooth to create the glaze.
10 - Transfer warm donuts to a wire rack. Drizzle glaze over each donut or dip tops directly into the glaze. Allow glaze to set for 20 to 30 minutes before serving.