Preheat the oven to 350 degrees.
Grease a 10-12 cup Bundt cake pan. Set aside.
In a stand mixer fitted with the paddle attachment, add the cake mix, pudding mix, vanilla, sour cream, cake flour, oil, egg whites, milk, and sugar. Mix on low for about 20 to 30 seconds, until most of the ingredients are moistened. Increase the speed to medium and mix for 2 minutes, scraping the bowl as needed. You can alternatively use a hand-held mixer.
While the batter is mixing, place the strawberry jam/puree in a separate medium bowl. Add a few drops of red coloring if desired. Set aside.
Gently stir the mixture together. A few streaks of the white batter is fine. After the batter has mixed for 2 minutes, pour half of the batter into the bowl with the jam.
Pour about ⅓ of the white cake batter into the bottom of the pan. Pour about ⅓ of the strawberry jam batter over the white batter. Continue this process until all of both batters have been layered.
Bake in the preheated oven for 35 to 40 minutes. Begin testing for doneness with a toothpick at 35 minutes, especially if your oven runs on the hotter side. Bake until the toothpick comes out with just a few crumbs attached.
Remove the pan from the oven and set it on a heat-proof surface; allow to cool for 20 minutes.
Invert the warm cake onto a cake stand/platter and allow it to completely cool if frosting.
The cake may be served warm if just a dusting of powdered sugar is used.
Any leftovers may be kept in an air-tight container at room temperature or refrigerated for 4 days. The cake may be frozen, well-wrapped without frosting for up to 3 months. Thaw then frost is previously frozen.