Note: In this step it’s possible to whisk the ingredients in the same order with a hand whisk instead of using a mixer. Mixing the ingredient slowly and with the paddle attachment in the mixer gives the cake a more ‘cakey’, light texture. In this case there's no need to mix for a few minutes, mix only until the ingredients are combined.
In a medium bowl using a mixer fitted with the paddle attachment, beat butter, oil, and sugar on medium speed for about 4 minutes, until light and fluffy. Scrape down the sides and bottom of the bowl as necessary. Add eggs and vanilla extract and beat until combined. On low speed, beat in flour mixture in 3 additions, alternating with the buttermilk in 2 additions, starting and ending with the flour. Mix in chocolate chips. Once all is added, be careful not to overmix the batter.