Heat mashed bananas in microwave about 3 minutes, stirring frequently (See Notes for alternative method). Put hot mashed bananas in a mesh strainer over a small bowl. Strain the pulp for about 15 minutes stirring frequently. There should be about a cup of strained pulp and ¾ cup liquid. Reserve the pulp. Pour the liquid into a small sauce pan, bring to a boil and reduce the volume by half. Pour concentrated liquid into a one-cup glass measuring cup and allow to cool.
Preheat oven to 350⁰F. Butter a 9-inch square pan.
Add buttermilk to banana liquid so liquid level measures ⅔ cup. Add vanilla and set aside.
Cream butter and sugar.
Blend eggs and banana pulp into butter/sugar mixture.
Whisk together flour, baking soda, and salt.
Add dry ingredients, alternating with banana liquid/buttermilk mixture. Mix until well blended. Pour into prepared pan.
Sprinkle nuts evenly over top of batter. Bake 40 minutes or until cake tests done.
An alternative method is to put bananas in a freezer bag, mash them flat, freeze solid and then thaw. This will also release the liquid which can then be concentrated. You can take advantage of the 1 or 2 too-soft-to-eat bananas by freezing the ripe banana pulp until enough saved to make bread. Putting sliced bananas in a food storage sized zip-loc bag (remove air) makes them easier and less messy to mash.