Place all the ingredients for the Ginger Chai concentrate in a small saucepan and bring to a boil.
Remove from heat and strain into a heat-proof jug.
Pick out the spice bits (pieces of ginger, cloves, cardamom, cinnamon, star anise) from the strained tea and add them to the liquid.
Allow to cool completely. Once cooled, strain the concentrate into a lidded glass jar/bottle and store in the fridge until ready to use.
Place mascarpone cheese, cream, sugar and rum in a large bowl. Whisk with an electric beater for a couple of minutes until fluffy and starting to form peaks.
To assemble the tiramisu, start with a 10-cup capacity (2.5 litre) round glass bowl with a flat base (alternatively use a round or rectangular ceramic baking dish).
Pour Ginger Chai concentrate in a separate shallow bowl.
Working quickly, dip the Savoiardi biscuits lightly in the chai concentrate and arrange them in the bottom of the glass bowl in a single layer.
Spread one third of the mascarpone cream mixture on top.
Dip more Savoiardi biscuits in the chai concentrate and layer them over the cream. Spread another layer of cream and dust this layer with cocoa powder.
Arrange the last layer of dipped Savoiardi biscuits. For the final cream layer, you can either spread it out in swirls or fill the remaining cream in a piping bag and pipe little mounds on top of the Savoiardi biscuits. Finish with a dusting of cocoa powder.
Cover and chill in the fridge for at least an hour or overnight.