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Preheat oven to 275 degrees. Line 16 muffin cups with paper liners and set pan aside.
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Mix the mashed banana with the lemon juice in a small bowl and set aside.
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Combine flour, baking soda, and salt in another small bowl and whisk to combine. Set aside.
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Cream the butter and sugar together in a large mixing bowl with an electric mixer until light and fluffy. Beat in eggs one at a time, beating well after each addition; beat in vanilla. With mixer set to low, beat in the flour mixture and buttermilk in three additions, beginning and ending with the flour. Stir in banana mixture until just combined.
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Divide batter between prepared pans and bake 26 to 30 minutes, until cake springs back when touched lightly with your fingertip. Cool cupcakes completely in pan for 5 minutes then remove to a wire rack to cool completely before frosting.