In a small cooking pot, add sugar (15g) and 1 tsp water and bring it to a boil. Once it starts to be lightly brown, then add the nuts and stir for a few seconds. Transfer over the baking sheet and let it cool. Set aside.
2. In an another small cooking pot add heavy cream (50ml) and heat until almost boiled. Remove from the heat and add white chocolate (40g), mix gently until the chocolate blocks are melted completely. Transfer into a heat-safe bowl and let it cool to room temperature then chill in the fridge.
3. Once the chocolate mix is cold : in a medium bowl, add mascarpone, sugar and the chocolate mixture from #2. Mix well. Add matcha sifting with a sieve. Mix well. Add the nuts from #1 and mix.
4. Prepare the silicone mould (here I used a 4cm diameter * 15-hole mould). Place the matcha cream in each hole. Flatten the surface, using a spoon. Place in the freezer until it is frozen. I usually make this one day before I want to make mochi.
5.Mochi time! Once the cream is frozen, start preparing the mochi mixture : In a microwave safe glass bowl, combine flour and sugar. Stir to combine. Add the water and mix with an egg beater until smooth. Cover the bowl lightly with cling wrap.
6. Microwave the mixture for 1 minute. Then stir the dough with a spatula. Put it back in the microwave again for another minute and stir lightly. Put it back in the microwave for another 30 seconds (not 1 min) and stir. And once more microwave for 30 seconds and stir. The mixture should be now very thick and translucent.
7. Transfer the mochi dough on your working surface floured with katakuriko/potato starch. As it is very hot you should use a spatula or a dough cuter. Cut the mochi into 15 equal pieces. Don't forget to coat your hand with katakuriko/potato starch.
8. With your hands, take one of these pieces and flatten it into a thick disc. Then put a frozen cream ball from the freezer in the middle of the mochi disc. Pull out the cream balls one at a time from the freezer as they will melt quickly. Pull the edges of the dough disc together to cover the cream ball. Press together the edges to seal well. Smoothen the mochi by rolling it on the dusted board.