Preheat your oven to 350 degrees Fahrenheit and grease two 9 inch round cake pans.
In a bowl, mash strawberries with a potato masher. You'll want to mash it to the point that most of it is liquid-y, but there are still strawberry chunks left. Set aside.
In a large mixing bowl, mix the flour, salt, baking powder, and baking soda together. Set aside.
Cream the butter and sugar together with a mixer for about 2-3 minutes. You want it light and fluffy.
Add the egg whites and mix again.
Add the sour cream and mix until fully incorporated.
Add half of the dry ingredients to the wet ingredients and mix just till all mixed. Do not overmix the cake batter.
Add the milk. Mix again (and don't overmix.)
Add the other half of the dry ingredients until fully incorporated.
Take 1/4 cup of mashed strawberries out of the strawberries you mashed earlier. Set that aside for the frosting. Add the rest of the strawberries to the cake batter and mix JUST until completely incorporated into the batter.
Pour half of the batter into one cake pan, and half into the other. If you want them the same size weigh them on a kitchen scale.
Pop them in the oven and cook for 33-35 minutes. They are done when you can stick a toothpick in them and it comes out with just a few crumbs, but no wet batter.
Allow them to cool for about 15 minutes before popping them out of the their pans to finish completely cooling.