Preheat the oven to 350°F (175°C). In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the egg and mix until combined. Sift in the flour, matcha powder, baking powder, and salt, then mix until a dough forms.

Scoop tablespoon-sized portions of dough onto a baking sheet lined with parchment paper, spacing them apart. Flatten slightly with a spatula.

Bake for 12-15 minutes or until the edges are lightly golden. Remove from the oven and let cool completely.

In a mixing bowl, whip the chilled heavy cream until soft peaks form.

Gradually add the powdered sugar, matcha powder, and vanilla extract, continuing to whip until stiff peaks form.

Once the cookies are completely cool, pipe or spoon the mousse filling onto the flat side of half the cookies.

Top with the remaining cookies to create sandwiches. Refrigerate for at least 30 minutes before serving.