In a saucepan, heat the heavy cream over low-medium heat. Watch for when it begins to simmer (tiny bubbles start forming around the pan’s edges), then remove the saucepan from the heat immediately to avoid boiling.
Pour the hot cream over the chocolate. Let it sit without stirring for about a minute to allow the chocolate to soften. Mix using a hand blender until shiny and smooth.
If you don’t have a hand blender, gently stir the mixture, starting from the center and working out to the edges. Combine until you achieve a smooth, glossy ganache. Keep the ganache warm.
Arrange the cookies on a tray. Transfer the warm chocolate ganache to a piping bag.
Fill the hollow top of the cookies with the chocolate ganache. Tap the cookies to level out the ganache filling.
Let the ganache set for 10 minutes before serving.