Preheat your oven to 325 degrees F.
In a medium sized mixing bowl, whisk together the flour, baking powder, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar for about 2 minutes, until light and fluffy. The mixture should be lighter in color and fluffier in texture.
Add the egg whites, one at a time, and mix well after each addition. (Be sure to scrape the sides and bottom of the bowl between each addition too). Add the raspberry extract or emulsion and vanilla.
With the mixer on low, add 1/3 of the flour mixture, followed by half of the raspberry puree and half of the buttermilk. Stir in another 1/3 of the flour mixture, the remaining raspberry puree and buttermilk and then the remaining flour. Mix until just incorporated.
Pour the cake batter on top of the Oreo crusts, about 16 ounces of batter in each pan. Bake the cakes for about 30 to 35 minutes, until a toothpick inserted in the center comes out with just a few moist crumbs on it.
Let cool in the pans for about 15 minutes before inverting them onto wire racks to cool completely.