Preheat the oven to 350°F. Line three 6-inch baking pans with parchment paper.
To make the batter: In the bowl of a stand mixer with the paddle attachment, (or alternatively, using a hand mixer, or large bowl with a whisk) add the cane sugar, canola oil, milk, buttermilk, eggs, and vanilla extract. Whisk together.
Add in the cake flour, baking powder, and sea salt in that order. Mix on low until combined into a smooth batter.
Transfer to the prepared baking pans evenly, about 400 grams each.
Bake for 35 minutes or until a toothpick inserted in the center comes out clean. Allow to cool completely in the pan.
To make the frosting: In the bowl of a stand mixer fitted with the paddle attachment (or using a hand mixer) add the butter, cream cheese, powdered sugar, cocoa powder, and melted chocolate. Mix on low until combined, then speed up to high and mix for 2 to 3 minutes or until light and fluffy.
Transfer the frosting to a piping bag with Ateco tip #827.
On a plate or cake stand, add a small dollop of frosting and place the first cake layer on it. This helps to prevent the cake from sliding.
Pipe a circle of frosting around the outer edge of the cake and spiral into the middle. Top with rainbow sprinkles.
Top with the second cake layer. Repeat this process with the other two cake layers.
Top the cake with nonpareil sprinkles.
Store in an airtight container in the fridge for up to 3 days.