For the perfect sandwich cookie, both the cookie and chocolate almond cream need to be the right texture. When baking the cookies, make sure the edges are set, but the centers are still soft. Chill the filling until thickened but not hard. Once the filling has set, it won’t melt at room temperature, but will be easy to work with and won’t squish out with that first big bite.
Make the cookies: Whisk the flour, cocoa powder, baking soda and salt in a medium bowl. Beat the butter, granulated sugar and brown sugar in a large bowl with a mixer on medium-high speed until fluffy, 2 to 3 minutes. Add the egg and vanilla; beat until smooth. Reduce the mixer speed to low, add the flour mixture and beat until just combined. Using a rubber spatula, stir in the finely chopped chocolate, making sure all the dry ingredients are combined.
Transfer the dough to a large sheet of parchment paper and pack into a firm log, 8 to 9 inches long and 1 1/2 to 2 inches wide. Wrap and refrigerate until very firm, at least 1 hour or overnight.
Position racks in the upper and lower thirds of the oven and preheat to 350˚ F. Line two baking sheets with parchment paper. Slice the log into 1/4- to 1/2-inch-thick rounds (about 20) and arrange 1 inch apart on the baking sheets. If any of the rounds are uneven, press and shape into an even round. Bake until the edges are set but the centers are still soft, 12 minutes. Let cool 5 minutes on the pans, then transfer the cookies to a rack to cool completely.
Meanwhile, make the filling: Combine the chocolate, almond butter and heavy cream in a small saucepan. Cook over medium-low heat, stirring, until the chocolate melts and the mixture is smooth, 3 to 5 minutes. Remove from the heat, stir in the vanilla and salt and set aside to cool.
Beat the butter and confectioners' sugar in a large bowl with a mixer on medium speed until fluffy, 2 to 3 minutes. Add the chocolate mixture and continue beating until smooth and lightened. Refrigerate until firm yet spreadable, about 1 hour.
Turn over half the cookies and spread or pipe 2 tablespoons filling on each, mounding the filling so it’s thicker to one side and forms an incline. (If the filling is too firm, let stand at room temperature to soften, then vigorously stir.) Top each with a second cookie so it sits at an angle. Fill in any gaps around the sides of the cookies with more filling. Add 2 candy eyeballs to each cookie, pressing them into the filling.