Preheat the oven to 350°F (175°C). Grease and flour an 8-inch round cake pan.
In a large bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition, then stir in 1 teaspoon vanilla extract.
In a separate bowl, whisk together the all-purpose flour, baking soda, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with the mashed bananas and sour cream, beginning and ending with the dry ingredients. Mix until just combined.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake in the preheated oven for 35 to 40 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
While the cake cools, prepare the glaze by mixing powdered sugar, milk, and 1/2 teaspoon vanilla extract in a small bowl until smooth.
Once the cake is completely cooled, pour the glaze evenly over the top and let it set for 10 minutes.
Divide the white fondant into two portions. Tint one portion with a few drops of pink food coloring and knead until evenly colored.
Roll out the white fondant and cut out kawaii-style eyes and mouth shapes. Roll out the pink fondant and cut small circles for cheeks and bows.
Use the black edible marker or black food coloring gel to add details to the eyes and mouth on the white fondant pieces.
Carefully place the fondant decorations onto the glazed cake to create a kawaii face with eyes, cheeks, mouth, and a bow.
Let the decorated cake set for an additional 15 minutes before serving.