Preheat oven to 350°F (175°C) and line two baking sheets with parchment paper.

In a large bowl, whisk together the melted butter, cocoa powder, and peanut butter until smooth and glossy.

Beat in the brown sugar until well incorporated, then add the egg and vanilla extract.

Add the flour, baking soda, and salt, stirring just until combined. Be careful not to overmix.

Scoop out approximately 1 tablespoon portions of dough and roll into balls.

Roll each ball in granulated sugar to coat completely.

Place the sugar-coated dough balls about 2 inches apart on the prepared baking sheets.

Bake for 8-10 minutes, until the cookies are set around the edges but still soft in the centers.

Remove from oven and let cool on the baking sheet for 5 minutes.

While cookies are still warm, press a malted milk ball onto the center of each cookie to create the spider's body.

Transfer melted chocolate to a zip-top bag and snip off a tiny corner. Pipe eight legs extending from each malted milk ball to create spiders.

Add tiny eyes using white and black icing or sprinkles.

Allow the chocolate to set completely before serving.