Berries give this snack cake a bright pop of color and fresh flavor.
Have you heard about our Young Chefs’ Club? Members get a themed (and kid-tested) box delivered each month!
Adjust oven rack to middle position and heat oven to 350 degrees. Spray inside bottom and sides of 8-inch square metal baking pan with vegetable oil spray. Line bottom of baking pan with 8-inch square piece of parchment paper.
In medium bowl, whisk together flour, baking powder, and salt.
In bowl of stand mixer (or large bowl if using handheld mixer), combine softened butter and sugar. Lock bowl into place and attach paddle to stand mixer, if using. Start mixer on medium-high speed. Beat until mixture is light and fluffy, 3 to 4 minutes. Stop mixer.
Use rubber spatula to scrape down sides of bowl. Add eggs and vanilla. Start mixer on medium speed and beat until combined, about 30 seconds. Stop mixer.
Carefully add half of flour mixture. Start mixer on low speed and mix until combined, about 30 seconds. With mixer running, slowly pour in milk and mix until combined, about 30 seconds. Stop mixer.
Add remaining flour mixture. Start mixer on low speed and mix until well combined, about 30 seconds. Stop mixer. Remove bowl from stand mixer, if using.
Use rubber spatula to scrape down sides of bowl and stir in any remaining dry flour (batter will be very thick). Add blueberries and raspberries and use rubber spatula to gently stir until just combined.
Use rubber spatula to scrape batter into parchment-lined baking pan and smooth top (make sure to spread batter into corners to create even layer).
Place baking pan in oven. Bake until cake is golden brown and toothpick inserted in center comes out clean, 30 to 35 minutes.