Preheat your fan oven to 160°C (325°F) or 175°C (350°F) if you're not using a fan oven. Line your cupcake pan with paper cases. Depending on their size, this recipw will yield 12 to 14 cupcakes.
In a medium bowl sift together flour, cocoa powder, baking powder and salt. Add sugars, whisk until combined and set aside.
In a small bowl or a measuring jug, whisk together eggs, vegetable oil, sour cream and vanilla extract. These are your wet ingredients.
In a small pot, bring your water to a boil or simmer, it's important that it's hot. Add the wet ingredients to dry ones, stir a little bit, it'll be somewhat dry. Then add the boiling water and whisk until combined. I usually add the water in two stages to minimize clumps forming.
Divide the batter evenly among the paper cases, filling them ¾ of the way full. Bake for 18-22 minutes or until they're springy on the top. You can poke one with a toothpick - if it comes out with a few moist crumbs, they're done.
When they're done, leave them in pan for 5-10 minutes to cool. Then transfer them to a wire rack to cool completely before frosting.
for earl grey milk:
In a small pot, bring milk to a simmer. Add loose leaf earl grey tea, cover with a small plate and let it steep for 15-20 minutes. Once it's done, strain it to get rid of tea leaves and leave it to cool to room temperature before using. You're going to end up with a little bit less milk than you've started with. Just add regular whole milk to reach 250 g.
for earl grey syrup:
Bring water to a boil, combine it with tea leaves and let it steep for 5 minutes. Once the tea is done, strain it a small saucepan and combine with sugar. Cook on a medium to high heat until it thickens a bit and form a syrup. Leave it to cool to room temperature before using. (You're not going to use all of it in a cake, but it's hard to make it with smaller quantities.)
for earl grey buttercream:
In a small saucepan combine flour, sugar, salt and earl grey milk. Cook over medium-high heat, stirring constantly with a whisk until it thickens.
Once it's done, transfer it to a clean bowl, put a cling film over it touching the surface to prevent "skin" forming and let it cool to room temperature. This is your pudding base. You can do this in a fridge, but before using it let it come to room temperature.
Using a stand mixer fitted with the paddle attachment, beat the butter for 5 minutes until it's pale and fluffy. Then add spoonful by spoonful of previously made pudding. Mix until it's all nicely incorporated and with no lumps. Add vanilla extract and mix until everything is combined.
Assembly:
Put the buttercream in a piping bag fitted with 2D tip (or whichever you prefer ? ) and frost the cupcakes. Drizzle them with earl grey syrup and sprinkle some chopped chocolate.They are best eaten at room temperature. You can keep them in an airtight container at room temperature for 2 days or longer in the fridge. Just let them come to room temperature before eating them. Fridge makes cakes and cupcakes drier so bear that in mind.
Notes
You can use a hand mixer if you don't have a stand mixer.
If you still don't own one, I advise you to buy a kitchen scale - they're cheap and easy to use, and always guarantee the same results in baking!
Not all ovens are the same. I suggest to check the cupcakes at 18 minute mark and then adjust the baking time accordingly. Get to know your oven!