**To Make the Cake:**

1. Preheat your oven to 350°F (175°C). Grease and flour two 8-inch round cake pans and line the bottoms with parchment paper.

2. In a large mixing bowl, combine the mashed bananas, brown sugar, applesauce, vanilla extract, and egg. Mix until well combined.

3. In a separate bowl, sift together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt.

4. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Be careful not to overmix.

5. Fold in the semi-sweet chocolate chips until evenly distributed throughout the batter.

6. Divide the batter evenly between the prepared cake pans.

7. Bake for 25-30 minutes, or until a toothpick inserted into the center of the cakes comes out clean.

8. Allow the cakes to cool in the pans for 10 minutes, then turn them out onto wire racks to cool completely before frosting.

**To Make the Chocolate Frosting:**

1. In a large bowl, beat the softened butter until creamy and smooth.

2. Gradually add the powdered sugar and unsweetened cocoa powder, beating until well combined.

3. Add the salt, vanilla extract, and heavy cream. Beat on high speed for about 3 minutes, until the frosting is light and fluffy. Adjust the consistency with additional cream or powdered sugar as needed.

**To Assemble the Cake:**

1. Place one cake layer on a serving plate or cake stand.

2. Spread a generous amount of chocolate frosting over the top of the first layer.

3. Place the second cake layer on top of the frosted layer.

4. Apply a crumb coat (a thin layer of frosting) over the entire cake to seal in crumbs. Chill the cake for 15 minutes to set the crumb coat.

5. Apply the remaining frosting evenly over the top and sides of the cake.

6. For an added touch, garnish with chocolate shavings or banana slices before serving.