Line two large baking sheets with parchment paper or silicone mats. In a mixing bowl, beat room-temperature butter and granulated sugar with an electric hand mixer for 2 to 3 minutes, until pale and fluffy.
Add the all-purpose flour and fold with a spatula just until a smooth batter forms. Do not overmix.
Transfer dough into a piping bag fitted with a medium round tip (or cut a corner opening). Pipe 2-inch strips onto your baking sheets, leaving 1 inch of space between each.
Bake at 180°C (350°F) for 12 to 15 minutes, until the edges are lightly golden. Let them cool completely on the pan.
Make the chocolate ganache
Chop chocolate finely and place in a bowl with the heavy cream. Microwave in 30-second bursts, stirring between each, until completely melted.
Let the ganache cool at room temperature or in the fridge for 15 minutes, until thick and spreadable. Spread or pipe ganache onto the flat side of half the cookies, then top with the remaining cookies to create sandwiches.
Notes
Measuring Flour: For the most accurate texture, weigh your flour with a kitchen scale. If using measuring cups, fluff the flour with a spoon, scoop it into the cup, and level it off with a knife without packing it down.
Chocolate Selection: Stick to chopped baking chocolate bars with 55% to 70% cocoa content. Avoid standard chocolate chips because they contain stabilizers that prevent a smooth melt.
Butter Temperature: Ensure your butter is softened to room temperature, not melted. Melted butter causes the cookies to spread thin and lose their piped shape in the oven.
Ganache Consistency: Let the warm ganache cool at room temperature or in the fridge for about 15 minutes to firm up into a spreadable texture before assembling. Warm ganache will run off the edges.
Uniform Piping: Pencil 2-inch guidelines onto the back of your parchment paper and flip it over before piping to make sure all your wafer pairs match up neatly.
Storage: Keep assembled cookies in an airtight container at room temperature for up to 3 days or in the fridge for up to 5 days.