In the bowl of a stand mixer, combine the cake mix, gelatin, milk, oil, eggs and fresh strawberry puree.
Beat on low speed until mixed, then beat on medium speed for 2 minutes.
Grease and flour two 9″ round cake pans (*see instructions below for 9×13 cake pan*).
Divide the batter evenly between the two cake pans.
Bake at 325 degrees for 29 minutes, or until the top of the cake bounces back when you touch it.
Allow the cakes to cool for 10 minutes, then turn them out onto a wire rack to cool completely.
For the Frosting
In the bowl of a stand mixer, cream the softened butter for 1-2 minutes until nearly white in a appearance.
Add 2 cups of powdered sugar and the vanilla extract. Beat on low for 1 minute. Scrape down the sides of the bowl.
Mix in the heavy cream* and another 2 cups of powdered sugar. Beat on low for 1 minute. Scrape down the sides of the bowl.
Add the last 2 cups of powdered sugar and the strawberry jam. Beat on low speed until mixed, then beat on medium speed for 1-2 minutes, until the frosting is creamy and smooth, scraping down the sides as needed.
To Frosting the Cake:
Place one cake layer on a cake plate.
Spread frosting on top of the first layer.
Place the second cake layer on top of the frosting.
Use the remaining frosting to frost the top and sides of the layer cake.
Optional: Decorate the top middle of the cake with a few tablespoons of strawberry jam and fresh strawberries.
Store at room temperature until serving, then store leftovers in the refrigerator.