01 -
Preheat oven to 175°C. Lightly grease a 23x33x8 cm baking dish with nonstick spray and set aside.
02 -
In a medium bowl, whisk together flour, baking powder, and baking soda until thoroughly blended.
03 -
In the bowl of a stand mixer fitted with the paddle attachment, or with an electric hand mixer, beat the butter, vegetable oil, and sugar for 3 minutes on medium speed until light and fluffy.
04 -
Add eggs one at a time, mixing well after each addition. Add the egg yolks and vanilla extract, blending until smooth and combined.
05 -
Add half of the dry mixture to the batter and mix on low speed until just combined. Pour in half of the buttermilk and blend until smooth. Repeat with remaining dry ingredients and buttermilk, scraping the bowl to ensure an even batter.
06 -
Transfer the batter into the prepared baking dish and smooth the surface. Bake for 35–40 minutes, or until a toothpick inserted in the center emerges clean. If excessive browning occurs, loosely cover with foil. Remove from oven and cool completely in the dish.
07 -
In a stand mixer or large bowl with a hand mixer, beat the butter until smooth. Gradually add cocoa powder and mix until uniformly incorporated.
08 -
Add powdered sugar in batches, beating thoroughly between additions until thick. Add salt, vanilla extract, and 60 ml of heavy cream. Continue mixing until the frosting is smooth and spreadable. If needed, add up to 15 ml more cream for desired consistency. Increase speed and whip for 3 minutes until fluffy, scraping down sides as necessary.
09 -
Spread frosting evenly over the cooled cake. Garnish with sprinkles if using. Slice and serve at room temperature.