Preheat oven to 350 degrees F and generously grease a round 8 or 9 inch baking pan. I always like to layer the bottom with parchment paper.
In a medium bowl, whisk the flour and baking powder together until well combined. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl using an electric mixer, beat the butter, oil, and 1 cup of sugar together on medium speed until smooth.
Add in eggs and vanilla extract. Continue to beat together until combined.
Add the flour mixture and alternate it with the buttermilk in 3-4 increments. Scrape down the sides of the bowl often.
Pour the cake batter into the prepared cake pan and smooth the top with a spatula. Bake for 30-35 minutes or when a toothpick inserted comes out clean (if you use a 8inch pan, bake for longer). The tops should be a light golden brown.
Let the cake cool for about 10 minutes, then run a butter knife along the edge of the cake pan and carefully flip it over on a cooling rack to cool for another 10-15 minutes or until cooled.
In the meantime, add the berries and 1 tablespoon of sugar to a small bowl and mix to combine. Let the berries sit until cake is ready to be assembled. Before adding the berries to the top of the cake, drain them or use a slotted spoon when transferring them over. The berries will create moisture and we don't want those juices on top of the cake.
Place the cake on a cake dish and spread the Cool Whip all over the top- this does not have to be an even layer or perfect! Add the berries and serve. Enjoy!