Preheat oven to 350 degrees an grease 8x8 baking pan.
Combine flour and soda and mix well.Blend sugar and butter and mix well.
Add sour cream,banana,egg and vanilla and beat until smooth.
add the flour mixture and blend until just combined.do not overmix.
Gently fold in the melted chocolate and stir just till marbled pattern appears.
Pour into baking pan and bake 40-45 minutes.
Ganache: scald cream in a saucepan. Remove from heat an ad chocolate. Cover pan with a lid and set aside.After 15 minutes stir frosting till smooth.Put in a bowl and refrigerate till firm about 30 minutes. Frost cake.