Preheat the oven to 180C/356F.

Melt the butter in a small pot or in the microwave. transfer the butter into a large bowl. 

Add the sugar and eggs to the bowl and stir well.

Peel the bananas and mash them with a fork.

Add the flour, salt, milk, and bananas to the bowl and mix well.

Dust the chocolate chips and add them to the cake mix. 

The dusting will prevent the chocolate chips from sinking to the bottom of the cake.

Dust the marshmallow and knife with some flour, cut the marshmallows into small pieces, and add them to the cake mix.  

Oil English cake baking tray. Pour the cake mix into the tray and bake in the oven for 45 - 50 minutes. Use a cake tester or a skewer to check if the cake is ready. Stick it in the center of the cake. If it comes out clean or with moist crumbs, the cake is ready. If it comes out with some wet batter, give the cake a few more minutes in the oven and check again. 

Let the cake cool down before trying to cut it. If the cake is not cold, it might fall apart when trying to cut it.