Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the egg and mix until well combined.

In a separate bowl, whisk together the flour, matcha powder, baking powder, and salt.

Gradually add the dry ingredients to the wet mixture, stirring until just combined.

Drop spoonfuls of the cookie dough onto the prepared baking sheet, spacing them about 2 inches apart.

Bake for 12-15 minutes or until the edges are lightly golden. Let cool completely.

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

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

Add the vanilla extract and gently fold to combine.

Once the cookies are completely cool, spread a generous amount of mousse filling on the bottom of one cookie and top with another cookie.

Repeat with remaining cookies and mousse. Serve immediately or refrigerate until ready to serve.