Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking pan or line it with parchment paper, leaving a little overhang on the long sides for easy brownie removal.

In a large bowl, whisk together the melted butter and sugar until smooth. Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In a separate medium bowl, sift together the flour, cocoa powder, baking powder and salt. This ensures a lump-free batter and an even texture.

Gradually add the dry ingredients to the wet mixture, stirring just until combined. If you're adding chocolate chips, fold them in now.

Pour the batter into the prepared pan and spread it into an even layer. Bake for 25-30 minutes, until a toothpick inserted into the center comes out with just a few moist crumbs.

Let the brownies cool completely in the pan on a wire rack, about 1 hour (or chill for 20 minutes to speed things up). Once cool, slice into 12 rectangles.

Top each brownie with a mini Twix bar.

Gently press two candy eyes into each mini Twix bar, toward the top of the bar.

Using the cookie icing, pipe a zigzag back and forth over each Twix bar to create the mummy's bandages, leaving the eyes peeking through. If your hand isn't very steady, it may be easier to transfer the icing to a piping bag for this step.

Set the brownies aside and let the icing firm up for about 15 minutes before serving your Halloween brownies!