Preheat oven to 350 degrees, and grease and flour a 13x9 inch pan (or line with parchment paper.) See footnote for how to grease and flour a pan.

Puree 1 ½ cups raspberries, set aside. If desired, strain pureed raspberries through a mesh strainer to avoid seeds in the cake, although this is not a necessary step.

In a large bowl, combine 2 ¾ cups flour, 1 ⅔ cups sugar, 1 teaspoon baking soda and 1 teaspoon salt.

In the same bowl, add 4 large egg whites, 1 cup sour cream, ¾ cup vegetable oil, 1 tablespoon lemon juice, and 1 teaspoon vanilla; stir until fully combined.

Fold in the 1 ½ cups pureed raspberries until combined. For a more vibrant "raspberry" color, add a tablespoon of beet juice for a natural boost in color, or your favorite deep red food coloring.

Pour batter evenly into greased and floured 13x9 inch pan.

Bake in a 350 degree oven for 22-35 minutes, until center springs back from touch, or toothpick inserted in center comes out clean.

Cool completely before frosting.

Add one stick of butter, completely softened, to a bowl. Add 1 ½ cups powdered sugar, ¼ cup heavy whipping cream, and 1 teaspoon vanilla.

Using an electric mixer (or a whisk by hand-but you'll need to use some elbow grease) beat the frosting ingredients until fluffy, about 2 minutes by mixer or 5 minutes by hand whisk.

Wait until cake is completely cooled to frost. Store frosting at room temperature until ready to frost the cake. Wait until the cake is fully cooled to avoid frosting runoff.

If desired, decorate the cake with fresh raspberries and sliced lemons.