Here's a recipe I found in Vegetarian Times that was easy to veganize . It has a great texture and is not too sweet. You can bake it in a 9 x 5 loaf pan or as 12 muffins. I don't think I mashed the banana enough but it was still really good.
Chocolate banana cake
1 1/2 cups whole wheat pastry flour (that's pastry flour, stirred well before measuring)
1/2 cup evaporated cane juice (like Sucanot )
1/3 cup unsweetened cocoa powder
1 teaspoon baking soda
1/4 teaspoon salt
almost 1 cup soy milk
3/4 cups mashed ripe banana
1/3 cup canola oil
2 teaspoons vanilla extract
1 tablespoon rice vinegar or cider vinegar
1/4 cup vegan chocolate chips for glaze (optional)
extra chocolate chips (see note)
1. Preheat oven to 350˚F. Coat pan with oil.
2. Sift together flour, sugar, cocoa, baking soda and salt in bowl.
3. Put vinegar in measuring cup and add soy milk to make one cup. Let stand for a minute to curdle. Whisk together soy milk , banana, oil and vanilla. Fold liquid mixture into flour mixture.
4. Put batter into prepared pan and bake 50 minutes for a loaf and 20 minutes for muffins., or until a toothpick inserted in center comes out clean.
5. Cool 5 minutes in pan. Remove from pan and cool on a wire rack.
6. If desired, melt 1/4 cup vegan chocolate chips in a small pot. Use a low flame and stir constantly (seriously, pay attention -it only takes a minute) until melted. Spread over the top of the cooled cake. This is a thin coating, more like a glaze.
note: This chocolate cake is relatively healthy with the whole wheat pastry flour and only 1/2 cup of sugar, 1/3 cup of oil and all that banana. And I'm not saying you should change that, but you could stir 1/2 cup or more of vegan chocolate chips into the flour mixture before you fold in the wet ingredients. This is entirely optional, but maybe for a special occasion...
Lazy cooks read this: My friend Claire put chocolate chips on top of the hot cake where they softened, and she spread them into an icing. Now, THAT'S easy. Yikes.