Chop the sugar-free white chocolate into very small pieces, by slicing it finely (or use chocolate chips).
Take from the total amount of cream 1 tablespoon, and mix with the matcha powder until it forms a paste.
Put the remaining cream in a small sauce pan on very low heat. Do not boil. When you notice the start of bubbling on the edges of the pan, remove from the heat, and drop all the chopped chocolate into the pan at once. Gently press the chocolate onto the cream with a spatula, but don't mix it yet.
After one minute, start mixing the chocolate into the cream with a folding motion, to completely coat the chocolate that's hasn't melted yet with the hot cream. Continue mixing slowly. If the chocolate doesn't melt fully, turn on the stove on low again but try to hold the sauce pan at some distance above the heat source, while mixing the chocolate, until it melts fully.
Add the matcha cream, and mix until combined.
When the ganache is fully mixed and stable, add the brandy, one teaspoon at a time, mixing just until fully incorporated.
Pour the ganache into the prepared chocolate mold, and let it settle on the counter for one hour before covering with plastic film and putting it in the refrigerator, on the lowest shelf.
After a minimum of 6 hours, unmold the chocolate, and, using a very sharp knife, cut it into squares. Wash and dry the knife before cutting every time.
Sieve matcha powder (or half matcha half powdered sweetener) over the squares, or roll the squares into the powder.
Keep refrigerated for up to 2 weeks. It's best to remove remove from the fridge 20 minutes before consumption.
Notes
Following the correct ratio of chocolate and cream is essential for this recipe to have the right texture, so I recommend weighting these ingredients with a kitchen scale.If the ganache separates, it's grainy or you can see small oily puddles, add a very tiny bit (one teaspoon at a time) of cold cream to the mixture, it should come back together.This recipe will fill a 4.5 in (11.5 cm) square mold, and the resulting chocolate will have a height of 0.6 in (1.5 cm). If increasing the recipe, use a larger mold to have a similarly shaped chocolate.