Prepare the Cake Batter
Preheat the oven to 350°F (175°C). Line a 4-inch cake pan with parchment paper.
In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.
Add the egg and vanilla extract, mixing until fully combined.
Mix and Bake
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with the milk. Mix until just combined.
Pour the batter into the prepared cake pan and smooth the top.
Bake for 22–25 minutes, or until a toothpick inserted in the center comes out clean.
Cool and Assemble
Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack.
Once cooled, level the cake top if needed and slice horizontally into 2–3 layers.
Make the Frosting
Beat the softened butter until creamy. Gradually add powdered sugar, vanilla extract, and heavy cream.
Continue beating until the frosting becomes light and fluffy.
Reserve 1/3 of the frosting for the outer layer and divide the remaining frosting into bowls for coloring.
Decorate the Bento Cake
Layer the cake with frosting between each layer. Apply a thin crumb coat and chill briefly.
Frost the outside smoothly with the reserved frosting.
Use colored frosting, piping bags, edible markers, sprinkles, or berries to create fun designs and messages.