Place chocolate chips in a small bowl. In a small saucepan, heat heavy cream over medium heat until steaming and bubbles start to form around the edges. Pour heavy cream over chocolate and whisk until chips are completely melted and smooth. Cover the surface directly with plastic wrap and refrigerate until firm, about 2 hours.
Preheat oven to 400 degrees F. Grease skull pan with cooking spray and dust with cocoa powder. In a large bowl, whisk together flour, cocoa, baking soda, and salt.
In a separate bowl, whisk butter, sugar, buttermilk, food coloring, vanilla, vinegar and egg yolk until smooth.
Add wet ingredients into dry and whisk until just combined. Divide batter evenly among skull cavities.
Using a tablespoon, portion out the chilled chocolate mixture into 6 balls. Place 1 chocolate ball in the center of batter in each skull. Bake until center springs back when lightly pressed, about 18 minutes.
Remove from the oven and invert cakes onto a baking rack or large platter. Place yellow candies in the eye sockets and serve immediately.
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