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Brownie Picnic Cake with Chocolate Buttercream Frosting

Prep Time 15 minutes
Cook Time 40 minutes
Total Time 55 minutes
Servings 20 -24 serving
Author Si Foster, A Bountiful Kitchen



  • 1/2 cup water heated
  • 3 oz unsweetened chocolate chopped
  • 1 cup Kerrygold unsalted butter cut into tablespoons at room temperature
  • 2 1/4 cups granulated sugar
  • 2 eggs
  • 1 cup shaken buttermilk
  • 2 teaspoons vanilla
  • 2 cups all purpose flour
  • 1 teaspoon salt
  • 1/2 teaspoon baking soda

buttercream frosting:

  • 1/2 cup Kerrygold unsalted butter cut into tablespoons
  • 2-3 oz unsweetened chocolate chopped and melted* or 1/3 cup unsweetened cocoa powder
  • 4 cups powdered sugar
  • 2 dashes of salt
  • 1 teaspoon vanilla
  • 2-3 tablespoons milk or cream


  1. Place rack in center of oven and preheat oven to 350 degrees.
  2. Microwave water for one minute in a bowl. Add 3 oz chopped unsweetened chocolate and stir with a fork or whisk until smooth. Set aside.
  3. Place room temperature butter in a mixing bowl or in the bowl of a stand mixer. Turn the mixer on and cream butter and sugar until smooth.
  4. Add eggs and beat for one minute. Add buttermilk and vanilla mix until smooth.
  5. Add the melted chocolate mixture to the wet ingredients.
  6. Add flour, salt, baking soda all at once to bowl. Pulse the mixer until the ingredients are incorporated. When the dry ingredients have disappeared, turn the mixer to medium and mix for one minute.
  7. Grease a 9x13 pan with cooking spray. Scrape the batter into the pan and bake for 35-40 minutes*

Prepare frosting while cake is baking:

  1. Place butter, cut into tablespoons in a large mixing bowl. Add melted chocolate or cocoa powder, beat for one minute.
  2. Add powdered sugar, salt and vanilla. Mix, adding in milk or cream until smooth. See recipe notes for tips.
  3. When cake is cooled completely, spread frosting on top and serve.

Recipe Notes

  • Kerrygold butter is softer than most butter because it has a higher butterfat content. It is usually quite soft when removed from the refrigerator. If the butter needs to be softened up a bit, remove from the foil packaging and place on a plate. Let sit at room temperature for about 30 minutes.
  • You may use unsweetened solid chocolate bars or unsweetened cocoa powder in this recipe. I prefer unsweetened solid chocolate to cocoa powder in this recipe. To melt the chocolate, place in a bowl and microwave for 15 seconds at a time until melted. Chocolate burns quite quickly in the microwave, so only cook for a few seconds at a time.
  • Baking time may vary on this cake. It is meant to be a dense, moist, brownie-like cake. It should rise up as a cake would, but not quite as much as a traditional cake. Sifting the flour will produce a lighter and taller cake. If you prefer the finished product to be taller, sift the flour and bake at 375 for about 25-30 minutes.
  • Because the cake is dense, it may sink a bit in the middle and have taller outer edges. No worries! I simply use a silicone spatula or wood spatula and press the outer edges down so they are even with the center of the cake for an even finish. Do this while the cake is hot out of the oven!