Heat the oven to 350°F. Butter and flour a 13-by-9-inch baking pan.
In a large liquid measuring cup, whisk together ½ cup of the milk, the 4 egg yolks, 2 whole eggs, oil, and vanilla.
In a large bowl or the bowl of a stand mixer, whisk together the flour, sugar, baking powder, and salt. With an electric mixer on medium speed, beat in the butter until the mixture looks sandy, about 1 minute.
Add the remaining 1 cup milk and beat until smooth, about 1 minute. The batter will be thick.
With the mixer running, add the egg mixture in 2 additions, scraping down the sides of the bowl with a rubber spatula as needed.
Increase the speed to medium-high and beat the batter for another 2 minutes.
Transfer the batter to the prepared pan and spread it evenly with an offset spatula.
Bake until the cake is puffed and light golden and a toothpick inserted into the center comes out with moist crumbs attached, 25 to 30 minutes. Transfer the pan to a wire rack and let cool completely, about 45 minutes.
To make the frosting, melt the butter and cocoa powder over low heat in a saucepan. Mixture will be thick. Transfer chocolate/butter mixture to a large mixing bowl.
With mixer on low speed, add powdered sugar, vanilla, and milk.Mix on medium to high speed for a few minutes until thick and well combined.
If the frosting is too warm, refrigerate for about 20 minutes, until just firm enough to spread. Add to a piping bag and pipe half the frosting into skull molds and freeze.
Invert the cake onto a serving platter, if desired or leave in the 9x13 pan. Spread the frosting evenly over the cake and top with chocolate skulls, if desired.