Peel bananas and place in a sealable plastic bag. Squeeze and flatten the bag to mash the bananas.
Freeze sealed bag of bananas until solid.
Thaw the bananas and pour into a fine mesh strainer placed over a small saucepan.
Strain the banana pulp for about 15 minutes, stirring occasionally and collecting the liquid in the pan. (approx. 1½ cups pulp and ¾ cup liquid.)
Set pulp aside and cook the liquid over medium heat until liquid is reduced by half.
Blend banana liquid with cocoa powder and set aside.
Preheat oven to 350°F.
Generously butter a 8½ x 4-inch loaf pan (or line the pan with parchment).
Cream butter and sugars until light and fluffy. Beat in eggs. Combine flour, salt and baking soda.
Blend dry mix and banana pulp into butter mixture. Stir in nuts.
Remove about a cup of batter and combine with cocoa mixture.
Spoon the cocoa batter alternately with the plain batter into the prepared loaf pan. Swirl batters together using a knife.
Bake for one hour or until center tests done. Cool on a rack for 15 minutes then remove from pan. Allow to completely cool before slicing.
Note: Freeze/thaw cycle releases liquid; alternatively the bananas could be heated in a microwave for 4 to 5 minutes to release the liquid before straining.