For the appropriate deep brown color and the distinctive flavor of the original Famous Wafers, it’s important to use Hershey’s Special Dark Cocoa Powder. These wafers work best when crushed, as for a cookie crumb crust or rum balls. In a cookie crumb crust, the recipe works as a 1:1 replacement for one packet of Famous Wafers. If using them whole in an icebox cake, allow extra time for softening. For best results, weigh your ingredients wherever a weight is given.

Whisk flour, cocoa, baking soda, and salt in medium bowl until combined. Combine sugar, water, and vanilla in second medium bowl and whisk until smooth. Add butter to sugar mixture and whisk until incorporated. Add flour mixture to sugar mixture and stir with spatula until fully combined.

Transfer dough to counter and knead until smooth, about 20 seconds. Roll into 5-inch log. Wrap tightly and refrigerate until very firm, at least 2 hours or up to 5 days.

Adjust oven racks to upper-middle and lower-middle positions and heat oven to 300 degrees. Line 2 rimmed baking sheets with parchment paper. Using sharp chef's knife, slice log into ⅛-inch-thick rounds and transfer to prepared sheets (20 cookies per sheet), leaving ½ inch between rounds. Bake until center of cookies gives very little when pressed with your fingertip, 20 to 22 minutes, switching and rotating sheets halfway through baking. Let cookies cool on sheets for 5 minutes, then transfer to wire rack and let cool completely. Serve. (Cookies can be stored at room temperature for up to 2 weeks.)