1 yellow cake mix, baked in a 15x10 jellyroll pan (line with parchment or wax paper, lightly grease paper) I've done this with 3 9-inch round pans as well and it works great!
Filling & Topping
2 cups strawberries (frozen work great for this recipe), unsweetened
6 tablespoons sugar, divided
¼ cup lemon juice
¼ cup flour
Bring strawberries, 4 tablespoons (¼ cup), and lemon juice to boil and cook for 5 minutes (on medium heat). Whisk 2 tablespoons sugar and ¼ cup flour together in saucepan. Add strawberry mixture and whisk together. Cook over medium heat for 5 minutes or until thick and bubbly. Pour into glass bowl, cover with plastic wrap, chill 2 hours.
Cut cooled cake in half. Place 1 cake layer on a serving plate; spread top with half of strawberry-lemon filling. Top with remaining layer. Spread remaining filling over top.
1 ½ cups whipping cream
6 tablespoons sugar
Beat cream and sugar on high until stiff peaks form. Spoon into a 1-gallon freezer bag. Press cream into one corner of bag, removing excess air. Snip a ¼-inch hole in corner of bag. Pipe cream vertically onto cake sides.