1. Grease a 9×13-inch cake pan and preheat oven to 275˚F (135˚C). This recipe uses a low and slow cooking temperature and time to prevent the cake from becoming dry.
2. Stir 1 ½ Tbsp vinegar or lemon juice into milk; let stand while preparing other ingredients.
3. Mix banana and 2 tsp lemon juice in a small bowl and set aside. In a medium bowl, combine flour, baking soda and salt; set aside.
4. In a large mixing bowl, cream together butter and sugar using a stand mixer fitted with the paddle attachment. Beat in eggs – one at a time and add vanilla. While mixing on low speed, add the milk mixture in 3 additions, alternating with the flour mixture. Once the mixture is evenly combined, add the banana mixture. Pour into prepared cake pan. Bake 1 -1 ½ hours (baking times vary depending on oven) until a toothpick inserted in the center comes out clean. Do not overbake. When cake is done, place it IMMEDIATELY in the freezer for 45 minutes – this is what makes it really moist.
1. Add cornstarch and salt to a small saucepan. Whisk milk into cornstarch and salt. Over medium heat, cook milk mixture, whisking occasionally – do not boil. Once mixture has thickened, remove from heat and cool to room temperature.
2. Meanwhile, cream butter and icing sugar until blended. When milk mixture is cool, slowly beat small amounts into butter mixture until completely mixed through. Add vanilla; mix thoroughly. If icing is too soft, chill slightly in the refrigerator then beat until it becomes a smooth, light spreading consistency.
3. Spread on the cooled cake and garnish with walnuts (optional). Store in the refrigerator. Before serving, let the cake stand at room temperature at least 15 minutes to allow the buttercream to soften and allow the cake to cut easily.