help with softer muffins?

[panur_links]

Yesterday I made some delicious banana-chocolate-chip muffins, and while they turned well, I'd like them to be more spongy/softer?

this is the recipe I used:

Recipe:

Banana Muffins
prep 10 min
bake 18 min
oven 400
makes 12 muffins

1 3/4 cups all-purpose flour
1/3 cup sugar
2 teaspoons baking powder
1/4 teaspoon salt
1 beaten egg
1/2 cup milk
1/4 cooking oil
3/4 cup mashed bananas (two whole bananas)
1/2 cup chocolate chips
1/2 t. baking soda

thank you for the help!