01 - Preheat oven to 350°F (175°C). Grease a 9x13-inch sheet cake pan and line with parchment paper if desired.
02 - Whisk flour, baking powder, baking soda, and salt together in a large bowl.
03 - In a separate bowl, beat sugar, melted butter, and eggs until smooth and slightly pale.
04 - Stir in vanilla extract, buttermilk, and soda. Mix until well combined.
05 - Add wet ingredients to dry ingredients. Mix gently until just combined, avoiding overmixing.
06 - Gently fold in blueberries using a spatula.
07 - Pour batter into prepared pan and smooth the surface.
08 - Bake for 28 to 32 minutes, or until a toothpick inserted in the center comes out clean.
09 - Allow cake to cool in pan for 10 minutes, then transfer to wire rack to cool completely.
10 - Combine powdered sugar, milk, and vanilla extract. Whisk until smooth and drizzle over cooled cake before serving.