Prepare cake as directed on box, except divide batter 2/3 into one pan and 1/3 into another.
Bake cakes, cool, remove from pan, and slice larger cake into 2 equal layers.
Whip the cream, sugar, and vanilla extract together until you get stiff peaks.
Roughly chop 2 cups of the strawberries, and neatly slice the rest for garnish.
Spread 1/2 strawberry jam on bottom layer of cake, followed by 1/4 whipped cream, and sprinkle a layer of chopped strawberries. Cover with middle layer and repeat.
Place top layer on cake, cover cake with remaining whipped cream, and garnish with sliced strawberries.