Preheat oven to 350F/180C.

In a large bowl cream together the butter and sugar until light and fluffy. Add the mashed banana and vanilla extract, and beat well.

Add the flour and salt and mix together. Stir in the chopped walnuts.

Use an ice-cream scoop or a tablespoon to scoop out the cookie dough and drop it onto the cookie sheet/s 2-3 cms apart.

Bake for 10-14 minutes or until it's light golden brown.

Remove the sheet/s from the oven and place it on a wire rack to cool completely before storing it in an air-tight container.