Preheat your oven to 350°F. Grease a 9x13" baking pan and line with parchment paper. Leave a bit of extra parchment paper hanging over the sides of the pan to make it easier to pull the cookie out later. Set aside.

In a medium bowl, combine flour, black cocoa powder, baking powder, and salt. Whisk to combine and set aside. - 2 cups (260g) all-purpose flour, 1 cup (80g) black cocoa powder, 1 teaspoon baking powder, ½ teaspoon salt

In a large bowl, combine butter and sugar. Use an electric mixer fitted with the paddle attachment to beat together until light and fluffy (about 2 minutes on medium speed). - 1 cup (225g) unsalted butter, 2 cups (400g) granulated sugar

Whisk the eggs in, one at a time, then mix in the vanilla. - 2 large eggs, 2 teaspoons pure vanilla extract

Add in the dry ingredients and mix together until you have a streak free cookie dough. It will be a bit soft and sticky.

Drop the sugar cookie dough into your prepared baking dish and spread into an even layer.

Bake in the center of your preheated 350°F oven for 25 to 30 minutes or until puffed and no longer shiny on the surface.

Cool completely to room temperature in the pan before decorating.

Add the semi-sweet chocolate to a heat safe bowl and set aside. - 12 oz (340g) chopped semi-sweet chocolate

Pour the heavy cream into a small pot. Heat over medium low until the edges of the cream just start to bubble - do not boil. - ¾ cup heavy whipping cream

Pour the hot cream over the chopped chocolate and let sit for 3 minutes. Mix until melted and smooth. It should all easily melt together, but if you have any undissolved chocolate, you can heat it in your microwave in 10 second bursts or over LOW heat on your stovetop to help melt it.

Allow the ganache to cool for about 5-10 minutes or until it's just slightly warm. While it cools, melt the white chocolate in your microwave or over a bain-marie***. - 3 oz (85g) chopped white chocolate

Now it's time to decorate! If any of the below steps aren't clear, you can see the above post for step by step photos to give you a visual. Click here to jump straight to the section.

Set aside about 1 tablespoon of the ganache to use for the ghost eyes and mouths then pour the rest of the ganache over your cooled cookie. Spread into an even layer.

Pour a little less than 1 teaspoon of the white chocolate onto the ganache. Use a spatula or knife to drag the white chocolate to create a streak that looks like a ghost tail. Repeat until you have multiple ghost shapes.

Dip the tip of a toothpick in the reserved ganache and use it to dot little ghost eyes and mouths. If there are any empty spaces in the ganache you want to fill, I like to sprinkle some crushed Oreo crumbs but you can just leave it as is if you prefer.

Chill the cookie for 20 to 30 minutes in the fridge or until the chocolate is set and you're able to pull the cookie out of the pan without the middle bending. Slice and serve!