Creating your own Bento Cake can be simple and enjoyable by following these steps:

1. Preheat your oven: Set your oven to 350°F (175°C). Grease and line a 6-inch round cake pan with parchment paper.
2. Mix dry ingredients: In a large mixing bowl, sift together the flour, sugar, cocoa powder (if using), baking powder, baking soda, and salt.
3. Cream the butter: In another bowl, cream the softened butter until light and fluffy. Add the eggs, one at a time, mixing well after each addition.
4. Combine wet ingredients: Add the buttermilk and vanilla extract to the butter mixture, mixing until well combined.
5. Combine mixtures: Gradually add the dry ingredients to the wet mixture, stirring until just combined. Do not overmix.
6. Bake the cake: Pour the batter into the prepared cake pan. Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean.
7. Cool the cake: Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.

Once the cake is cooled, you can begin assembling your Bento Cake.