How to Make It
Preheat oven to 350°F. Line a half-size (18- x 13- x 1-inch) rimmed baking sheet with parchment paper; lightly grease parchment with cooking spray.
Beat butter in a large bowl with an electric mixer on medium speed until creamy. With mixer running, gradually add sugar, and beat until light and fluffy, about 3 minutes. Add eggs, 1 at a time, beating until well combined after each addition. Beat in lemon juice and vanilla.
Stir together flour, gelatin, baking soda, and salt in a separate bowl. Add flour mixture and buttermilk alternately to butter mixture in 5 additions, beginning and ending with flour mixture, beating on low speed just until blended after each addition. Stir in 1 cup chopped strawberries. Spread mixture in prepared baking sheet. Bake in preheated oven until a wooden pick inserted in center comes out clean, about 30 minutes.
Cool cake completely on baking sheet on a wire rack, about 1 hour. Finely chop remaining 1/2 cup diced strawberries and stir into vanilla frosting. Spread cooled cake with frosting. Refrigerate at least 1 hour and up to overnight. Garnish each piece with halved strawberries.