Step 1: Preheat the Oven

• Preheat your oven to 350°F (175°C). Grease and line an 8×8-inch baking pan with parchment paper.

Step 2: Make the Chocolate Cake

• In a large bowl, mix together the flour, cocoa powder, sugar, baking powder, baking soda, and salt.
• Add the milk, vegetable oil, egg, and vanilla extract, and mix until well combined.
• Carefully stir in the boiling water until the batter is smooth (it will be thin).
• Pour the batter into the prepared baking pan and spread evenly.

Step 3: Bake the Cake

• Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
• Remove from the oven and let cool completely in the pan.

Step 4: Prepare the Oreo Filling

• In a medium bowl, beat the softened cream cheese until smooth.
• Gradually add the powdered sugar, mixing until well combined.
• Fold in the crushed Oreo cookies and whipped topping until everything is evenly distributed.

Step 5: Assemble the Cake Bars

• Once the chocolate cake has cooled, spread the Oreo filling evenly over the top of the cake.
• Place the cake in the refrigerator to chill while you prepare the ganache.

Step 6: Make the Chocolate Ganache

• In a small saucepan over low heat, combine the chocolate chips and heavy cream. Stir until the chocolate is melted and the mixture is smooth.
• Remove from heat and let cool slightly.

Step 7: Top the Bars

• Pour the chocolate ganache over the Oreo filling, spreading it evenly.
• Before the ganache sets, sprinkle with Halloween-themed decorations and additional crushed Oreos, if desired.

Step 8: Chill and Cut

• Refrigerate the cake bars for at least 2 hours to allow everything to set.
• Once set, lift the bars out of the pan using the parchment paper and cut into squares.