Beat the cake mix and next 7 ingredients at low speed with an electric mixer 1 minute. Scrape down the sides, and beat at medium speed 2 more minutes, stopping to scrape down sides, if needed. Strawberries should be well blended into the batter.
Pour batter into 3 greased and floured 9-inch cake pans.
Bake at 350° for 23 minutes or until cakes spring back when lightly pressed with a finger.
Cool in pans on wire racks for 10 minutes. Remove from pans; cool completely on wire racks.
Spread Strawberry Buttercream Frosting between layers and on top and sides of the cake. Garnish, if desired. Serve immediately, or chill up to 1 week.
To Make Buttercream: Beat butter at medium speed with an electric mixer 20 seconds or until fluffy. Add powdered sugar and chopped strawberries, beating at low speed until creamy. (Add more sugar if frosting is too thin (from juicer strawberries, or add strawberries if too thick.)