Preheat oven to 325 degrees F.
Butter two 9-inch round cake pans then line bottom with a round of parchment paper, butter parchment and dust lightly with flour.
Mash bananas with 1 tsp lemon juice.
In the bowl of an electric stand mixer fitted with paddle attachment, whip together butter, 2 Tbsp vegetable oil, granulated sugar and light brown sugar on medium-high speed until pale and fluffy.
Stir in remaining 2 Tbsp vegetable oil then blend in eggs one at a time.
Stir in vanilla and mashed bananas.
With mixer set on low speed, slowly add flour mixture alternating with buttermilk in 3 separate batches, beginning and ending with flour mixture and mixing just until combined after each addition.
Whip together cream cheese and butter until smooth and fluffy on medium-high speed, about 2 minutes, occasionally scraping down sides and bottom of bowl.
Add in powdered sugar and vanilla and whip on medium speed, about 2 minutes longer.
Add half of whipped cream into cream cheese mixture and gently fold until combined, then fold in remaining half. Spread over cake and keep cool.
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