Preheat the oven to 180 degrees Celsius (350 Fahrenheit, 160 degrees fan oven).
Add the butter to a pan set over a low heat, and melt.
Remove from the heat and allow it to cool slightly.
Add the butter to a large bowl together with the brown and caster sugar.
Use a hand mixer to beat them well, then add the egg and egg yolk and beat again to a get a smooth paste.
Sift in the flour, cocoa powder and bicarb of soda, then the vanilla extract.
Use a spatula to mix again to get a thick paste - if the dough looks too soft, set it aside for 5 minutes to harden, do not add any more flour.
You can either add some of the sprinkles now, or after they are out of the oven, in case some sprinkles might melt.
Use your hands to shape balls the size of a gold ball.
Line a baking tray with non-stick paper, and arrange no more than 6 balls, depending on the size of the tray - the cookies will spread in the oven, so they need plenty of room between them.
Bake for 12 minutes, then take them out of the oven, add some more sprinkles or edible eyes while they are still soft, by pressing gently so they can stick to the cookies.
Repeat with the rest of the cookies.
Allow them to cool down completely before storing into an air-tight container.