Preheat oven to 350°F. Grease and flour a 9 x 13-inch pan.
Mash bananas with a fork, set aside.
In a large bowl, combine cake mix and baking soda. Stir to combine. Add bananas, softened butter, buttermilk, eggs and vanilla. Beat at low speed until moistened (about 30 seconds). Beat at medium speed for 4 minutes.
Add coconut and pecans. Pour into prepared pan. Bake in the preheated oven for 30 to 35 minutes, or until center of cake springs back when lightly tapped.
Cook in glass bowl in microwave until reaches 240°F on candy thermometer. Beat and cool, beat several times while cooling. Poke holes in cake and pour some on while still hot. Continue to cool the rest and when cool and thickened, spread it on top of cake.Servings: 8 to 12