Exciting news: I'm going to be making Jessica and Judy's wedding cake in September! Apparently my previous wedding cake offering went over pretty well. They requested Earl Grey as a flavor component, which opens up all sorts of intriguing directions.
This frosting is at least as interesting for being a custard-based German buttercream as for being Earl-Grey-flavored. So far I've tried it dolloped on some rhubarb coffee cake and in the traditional frosting role on a lemony layer cake, and it's certainly tasty (although using only one tea bag gives it a very mild flavor). The texture is odd, though. At room temperature, it's almost like pudding, which is essentially what it is. But at fridge temperature, it looks weirdly grainy. Conclusion: could totally work as a filling with more tea, not so great for an outer coat.
This is exactly a half-recipe of the frosting here. Makes 1 quart.
1 c milk
1 bag Earl Grey tea
5 oz sugar
3/4 oz cornstarch
1 egg + 1 egg yolk
1/2 t vanilla
2 sticks (8 oz) salted butter, completely softened
1 pack (8 oz) mascarpone
2 t Meyer lemon juice
Heat the milk and tea bag together over medium heat until simmering, then turn off the heat, cover, and let steep for 3.5 minutes. Squeeze out the tea bag and discard.
While the tea is steeping, whisk together the sugar, cornstarch, egg components, and vanilla. Pour a few tablespoons of the hot milk into the egg mixture and whisk to combine. With the milk over low heat, drizzle the warm egg mixture into the milk, whisking constantly. Continue whisking for a few minutes until the custard thickens completely. Transfer to a mixing bowl, cover with plastic wrap, and chill in the fridge for at least half an hour while the butter finishes softening.
Using an electric mixer, beat the butter then the mascarpone into the custard 1-2 T at a time, making sure each addition is completely incorporated. Beat in the lemon juice. Use, refrigerate, etc.