Preheat your oven to 350°F (180°C). Grease a 9x13-inch baking pan. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream butter and sugar together at medium-high speed until light and fluffy, about 3-4 minutes.
Add eggs, mashed bananas, buttermilk, and vanilla extract. Mix at low speed until combined.
In a small bowl combine flour, baking soda, and salt.
Add flour mixture in three additions to the wet mixture, mix just until combined.
Pour batter into the baking dish and bake for 20-25 minutes. When the cake is done a skewer inserted in the middle should come out clean with only a few crumbs attached. Let the cake cool down completely before frosting it.
To make the frosting, beat together butter and confectioners' sugar in the bowl of a stand mixer fitted with the paddle attachment until light and fluffy.
Add vanilla extract and mascarpone and beat until blended. Frost cooled cake.