In a large bowl combine the melted butter, ¼ cup granulated sugar, brown sugar and vanilla. Add the egg and cookie butter and whisk until incorporated.
Add the flour, cornstarch, baking soda, baking powder and salt. Mix until combined.
Line a tray with parchment paper. Use a medium cookie scoop to portion the dough into about 22 balls, approximately 2 to 2½ tablespoons each. Refrigerate for 30 to 45 minutes.
Preheat the oven to 350°F. Roll the chilled dough balls in the ½ cup granulated sugar and arrange about 2 inches apart on parchment-lined cookie sheets. Bake for 10 to 11 minutes, until the edges are just set and the centers still look slightly underdone. If desired, gently reshape the hot cookies with a round cookie cutter as soon as they come out of the oven.
Let the cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.
Line a cool tray or work surface with wax paper. Melt the white wafers according to the package directions, stirring until smooth.
Spoon a little more than 1 tablespoon of melted coating onto the wax paper. Gently press the bottom of a cooled cookie onto the coating, allowing it to spread underneath and just reach the edges. Repeat with the remaining cookies, working a few at a time.
Drizzle the tops with additional melted coating and immediately sprinkle with crushed Biscoff cookies. Allow the coating to set completely before peeling the cookies off the paper and storing.