In a small bowl, mix together gelatin powder and 3 tbsp cream. Leave it to bloom for 10 minutes.
In a chilled mixing bowl, add 1 cup heavy cream. Sift in the powdered sugar and culinary grade matcha.
Whip to soft peaks. Set aside to rest on the counter.
In another bowl, melt the white chocolate chips in the microwave in 15-30 second bursts at full power, stirring between each increment. Continue this process for up to 90 seconds, stopping when the chocolate flows smoothly and has a shiny appearance.
Allow the melted white chocolate to cool for about 5 minutes, until it is warm to the touch.
While you wait for the chocolate to cool, heat the gelatin/cream mixture in the microwave for 5-10 seconds until the gelatin is dissolved. Let it cool until it is room temperature but still liquid.
See Note 2. Add the gelatin/cream mix to the melted chocolate. Mix with a spatula until incorporated.
Now, take your spatula and add one scoop of the whipped cream. Gently mix into the chocolate.
Gently fold in the rest of the whipped cream, adding 1/3 at a time. The mousse should be smooth and airy.
Fill a piping bag with the mousse.