Preheat the oven to 350°F and line an 8-inch square pan with parchment paper.
In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt.
Add the oil, egg, buttermilk, and vanilla extract to the dry ingredients. Stir until the batter is smooth and no dry streaks remain.
Pour the batter into the prepared pan and bake for 25 to 28 minutes, or until a toothpick comes out with a few moist crumbs. Let the cake cool completely in the pan.
Place the chocolate chips in a heatproof bowl. Heat the heavy cream in a small saucepan until it just begins to simmer, then pour it over the chips. Let it sit for one minute before stirring until the ganache is glossy and fully combined.
Spread the ganache evenly over the cooled cake. While the ganache is still soft, break the chocolate graham crackers into pieces and arrange them on top to form a simple haunted house shape. Let the ganache set at room temperature for 30 minutes before slicing.