15 sheets graham crackers (5" x 2.25" graham crackers or 2 cups graham cracker crumbs)
1/4 cup granulated sugar
2 tablespoons packed dark brown sugar
1/2 teaspoon baking soda
1/4 teaspoon fine salt
6 tablespoons unsalted butter, melted and cooled
1 large egg, lightly beaten
1/2 teaspoon vanilla extract
Arrange a rack in the middle of the oven and heat to 350°F. Line a baking sheet with parchment paper.
Process the graham crackers in a food processor fitted with the blade attachment until they are completely broken down into crumbs and have a sandy texture. Transfer to a large bowl.
Add the sugar, brown sugar, baking soda, and salt and whisk to combine. Add the butter, egg, and vanilla, then stir together with a rubber spatula until just combined into a soft dough.
Using a spoon or cookie scoop, measure 1 1/2 tablespoons of dough from the bowl. Roll between the palms of your hands to form a ball, then place on the baking sheet. Repeat with forming the remaining dough, spacing the dough balls about 2 inches apart.
Bake until the cookies have spread and the edges are lightly browned, 8 to 10 minutes. Let the cookies cool on the baking sheet for 5 minutes, then use a flat spatula to transfer to them to a wire rack to cool completely.