• Preheat the oven to 350 degrees Fahrenheit. Grease two 8 inch round cake pans with vegetable oil, line bottoms with parchment paper and then grease the parchment paper with vegetable oil as well.
• Make sure you pick through your 4 cups of mixed berries, wash and dry them. You want them to be as dry as possible because the raspberries will ooze liquid.
• Wash the strawberries and dry them. Depending on the size of the 2 cups of strawberries you have, cut them into quarters if they’re small, or eighths. You basically want the strawberry slices close in size to the other berries. 
• In a large bowl, whisk together the 2 ¼ cups of multipurpose gluten free flour, 1 cup granulated sugar, 1 ½ teaspoons baking powder, ½ teaspoon baking soda and ½ teaspoon kosher salt. Set aside.
• In a medium bowl, whisk together the 1 ¾ cups of milk , the ½ cup light brown sugar, the ½ cup plus 2 tablespoons vegetable oil, the 1 teaspoon white vinegar and the 2 teaspoons pure vanilla extract. 
• Slowly add the milk mixture into the flour mixture and use a spatula to combine as it gets thicker. It will still be fairly liquidy.  
• With a spatula, scoop half the batter into one pan and the other half into the other pan. Smooth the tops with a butter knife.
• Bake the cakes in the center rack at 350 degrees Fahrenheit for 35 minutes until golden on top and slightly springy when carefully touched.

  Let them cool in the pan and then transfer to a cooling rack. 
• Once the cakes have cooled completely, we are going to cut them in half horizontally. Now we will have 4 thin layers of cake.

  If the tops of your cakes have domes, you can very carefully slice them off to make the tops flat.

Make The Frosting

• In a large bowl of an electric mixer or using a hand held mixer, beat the ¾ cup of unsalted butter until it is smooth and creamy. About 2-3 minutes.
• Beat in the 6 ¾ cups confectioners' sugar, 3 teaspoons of vanilla and 1 pinch of kosher salt.
• Slowly add in the water a little at a time until you get the desired consistency. You may not need to use all the water.

  Divide the frosting into 4 equal servings.

Assemble The Cake

• Place one layer of cake on a cake plate/stand. Scoop ¼ (about 1 cup) of frosting on top of the layer and use an icing spatula to spread it around.
• Take 1 cup of all the mixed berries and scatter them all over the top of the buttercream.

  Place another layer of cake on top; repeat this same process. Do this with 3 layers of cake.
• Once you get to the top layer of cake, place it bottom (or flat) side up so the top of the cake is nice and flat. 
• Use an icing spatula to frost the top with the last 1 cup of frosting. Add another cup of mixed berries all over the top or you can put a mound of berries in the center of the cake as I did.
• Refrigerate the cake immediately so the berries don’t ooze as much liquid. 
• Keep the cake in the fridge until about 5-10 min prior to serving. If you serve it right away, the berries bleed when you slice into the cake.