Mix one well-ripened banana with vanilla ice cream and milk in blender until creamy; decorate with colored straws.
Butter cake pan and line bottom with parchment paper. Preheat oven to 175°C/350°F. Whisk together flour, baking powder, and salt and set aside.
Whisk together cocoa powder, some of the milk, and some of the sugar until smooth.
Beat eggs and sugar together until smooth. Stir in oil, some of the milk, and vanilla extract.
Fold in dry ingredients and mix until smooth.
Transfer approx. half of batter to chocolate mixture and stir until combined and uniform. To create stripes, first spoon about 2 tablespoons of vanilla batter into center of cake pan. With a clean spoon, scoop same about of chocolate batter directly onto center of vanilla batter. Continue alternating batters in this way until all of it is used. Bake for approx. 35 – 40 min., or until toothpick inserted into center of cake comes out clean. Let cake cool for 15 min. before turning out of pan. Let cool completely on cooling rack.
In a large bowl, cream some of the butter and cocoa powder until smooth and uniform. Stir in half of powdered sugar and salt, then add milk. Beat mixture on high speed, then add second half of powdered sugar. Beat until light and fluffy. Stir in some of the vanilla extract.
Frost cooled cake with buttercream and over in Smarties by gently pressing the candies into buttercream.