TO MAKE THE CAKE: Preheat oven to 350°. Prepare a 9-inch springform pan: grease sides and bottom, line bottom with parchment paper, grease again, then dust bottom and sides with white rice flour.
In a blender, puree the bananas with the sour cream until smooth. Add the eggs, lemon zest, and vanilla and blend. Put baking mix and sugar into large bowl of an electric stand mixer with paddle attachment and mix to combine on low speed. Add butter and half the banana mixture and mix slowly until dry ingredients are well moistened. Increase speed to medium and beat for one and a half minutes to aerate batter. Scrape down sides of bowl. Add remaining banana mixture in two parts, beating for 20 seconds after each. Scrape down sides of bowl. Scrape batter into prepared pan, smooth with spatula, and bake in center of oven for 35 to 45 minutes until a toothpick inserted in center comes out clean. Allow cake to cool in pan for 10 minutes before releasing springform to remove cake. Cool completely on rack before frosting.
TO MAKE FROSTING: In the bowl of electric stand mixer, mix all frosting ingredients on medium for 1 to 2 minutes or until smooth. If frosting is too thick, add more water one teaspoon at a time. Frost on cooled cake.
VARIATION: Cake is also delicious with chocolate ganache or simply dusted with powdered sugar.
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