Preheat the oven to 170C/325F. Lightly grease a donut pan.
Stir the lemon juice or vinegar into the milk and set aside to thicken slightly into ‘buttermilk’.
Add the Trex and sugar to a large bowl and beat together with an electric mixer until smooth and fluffy (about 2 minutes). Add the milk mixture and vanilla, then sift in the flour, cocoa powder, salt, baking powder and soda, then mix until combined.
Fill the doughnut moulds with the batter until 3/4 full and bake for 12 minutes, or until springy.
Leave to cool in the pan for 5 minutes, then carefully transfer to a wire rack to cool completely.
For the Earl Grey icing
Heat the milk and tea bags in a small pan until hot.
Remove from the heat and allow to cool and steep for 10 minutes. Gently squeeze the bags into the milk to extract more Earl Grey flavour, taking care not to tear the bags.
Allow to cool completely, then beat the infused milk with the icing sugar until smooth. If necessary, add a little more milk or icing sugar to get a thick drizzling consistency.
Drizzle the icing over the cooled cupcakes.
Best enjoyed on the same day or stored covered at room temperature for up to 3 days.