Put the tea into a teapot and add the boiling water. Set aside for 10 minutes.
Place the butter, sugar and vanilla in the bowl of an electric mixer fitted with the paddle attachment and beat for about 5 minutes, until pale and creamy. Add the eggs, one at a time and beat well after each addition. Add the flour, mixed spice and yoghurt. Mix on low speed until just combined. Strain the tea and add the liquid to the batter. Mix until just incorporated. Spoon into the prepared pan and bake for 32 - 35 minutes, or until a skewer inserted into the middle comes out clean. Allow to cool in the pan for 5 minutes before turning out onto a wire rack to cool completely. Dust with sifted icing sugar before serving.