Preheat your oven to 325 degrees F. Spray a 10-cup or larger bundt pan with nonstick spray and dust with sugar to cover the pan. Set aside.
In a medium sized mixing bowl, whisk together the cake flour, baking powder and salt. Set aside.
In the bowl of an electric mixer, fitted with the paddle attachment, beat the butter and the sugar for 3 to 5 minutes on medium-high speed. The mixture should be light and fluffy in texture.
Scrape down the sides of the bowl and begin to add the egg whites on medium speed, scraping down the sides and bottom of the bowl between addition, continue beating on medium-high speed until the texture is smooth and the volume is nearly doubled, about 3 to 5 minutes.
Add the vanilla and mix until incorporated.
With the mixer on low speed, add 1/3 of the flour mixture, followed by 1/2 cup of the buttermilk. Repeat the additions, 1/3 flour mixture, 1/2 cup buttermilk and the final 1/3 flour mixture. Only mix until the flour mixture is incorporated, about 30 seconds. Scrape down the sides and bottom of the bowl and mix again for another 20 to 30 seconds on low. Fold in the sour cream.
Pour half of the batter in the bundt pan. Carefully dollop the raspberry jam on the batter. Pour the remaining batter on top of the jam. Bake the cake for 50 to 60 minutes. (Mine took 55 minutes).
Remove from oven and let cool completely in the bundt pan before removing.