• Preheat oven to 350℉. Spray a 9x13 pan with non-stick cooking spray and set aside.
• In a large bowl or bowl of a stand mixer with a paddle attachment, add flour, sugar, cocoa, baking powder, baking soda, and salt. Whisk to combine.
  2 cups all-purpose flour, 1½ cups granulated sugar, ¾ cup dutch-processed cocoa powder, 2 tsp baking powder, 1½ tsp baking soda, 1 tsp salt
• In a separate bowl, whisk together the buttermilk, oil, eggs, and vanilla. Pour this into the dry ingredients and mix until combined.
  1 cup buttermilk, ½ cup vegetable oil, 2 large eggs, 2 tsp vanilla extract
• Carefully add in hot water and mix until fully combined.
  1 cup hot water
• Pour into the greased pan and bake for 30-35 minutes, or until the top is set and a toothpick inserted into the middle of the cake comes out clean or few crumbs.
• Allow the cake to cool completely on a wire cooling rack.

Make buttercream

• Add the blueberries to a saucepan and heat over medium heat for about 5 minutes, heating until the blueberries soften. Mash them with a fork or masher to break them up. Place them in the fridge to cool while making the rest of the frosting.
  ½ cup blueberries
• In a large bowl or bowl of electric mixer with a paddle attachment, mix the softened butter on high for 2-3 minutes, or until light and fluffy.
  1 cup butter
• With the mixer on low, add in the powdered sugar, 1 cup at a time. Add in the blueberry puree, vanilla, and salt, mixing until combined, scraping down the sides of the bowl as needed. Turn the mixer to high and beat for another 1-2 minutes until light and fluffy. If needed, add more powdered sugar to make the frosting thicker. If too thick, add a tsp of milk or cream.
  3 cups powdered sugar , 1 tsp vanilla extract, pinch of salt, ½ cup blueberries
• Frost the fully cooled cake and slice into squares to serve.