Begin by melting 150 grams of butter using a microwave or a saucepan over low heat. Allow the butter to cool to room temperature.

In a mixing bowl, combine the cooled melted butter with 200 grams of granulated sugar and 50 grams of dark brown sugar. Whisk until combined. Add in one egg, one egg yolk, and one teaspoon of vanilla extract, mixing until blended.

In another bowl, combine 260 grams of all-purpose flour, 40 grams of cocoa powder, ¾ teaspoon of baking powder, ¾ teaspoon of baking soda, and ¾ teaspoon of salt. Stir thoroughly and fold into the wet mixture until just combined.

Cover the dough and refrigerate for 30 minutes. Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.

Use a medium scoop to form dough balls and place them on the prepared baking sheet, flattening them slightly.

Bake the cookies for 9 to 10 minutes until the edges are set but the centers remain soft. Cool on the baking sheet for about 3 minutes before transferring to a cooling rack.

Prepare the vanilla buttercream by beating together 50 grams of room temperature butter, 80 grams of powdered sugar, ½ teaspoon of vanilla extract, and 1½ teaspoons of milk until light and fluffy.

Decorate the cookies with buttercream in zigzag lines and place candy eyes on top.