Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
Puree the strawberries in a blender until smooth. You'll use this both in the cake batter and the buttercream. This should yield about 2½ cups of strawberry puree.
In a medium bowl, whisk together the flour, baking soda, baking powder, and salt. Set aside.
In a large bowl, cream the butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Beat in the eggs one at a time, then stir in the sour cream and vanilla.
Gradually mix in the dry ingredients, alternating with the milk. Start and end with the dry ingredients. Mix until just combined.
Gently fold in 1½ cups of the pureed strawberries.
Divide the batter evenly between the prepared pans. Bake for about 30 minutes, or until a toothpick inserted into the center comes out clean. Let the cakes cool in the pans for 10 minutes, then transfer to a wire rack to cool completely.
In a large bowl, beat the butter until smooth. Gradually add the powdered sugar, 1 cup at a time, alternating with small amounts of the strawberry puree (1 cup of strawberry puree in total). Beat in the vanilla extract and a pinch of salt. Adjust the consistency with more powdered sugar or strawberry puree as needed.
Once the cakes are cool, cut the top part if the cakes have a big dome, then spread a layer of strawberry buttercream on top of one cake layer. Place the second layer on top and spread the remaining buttercream over the top and sides of the cake.
Garnish with fresh strawberries if desired. Slice and serve!