Recipe: Coffee Cake by James Hoffmann

The speaker first tested various forms of coffee in the cake sponge:

Coffee Type	Result	Rationale
Cheap Instant Coffee (Nescafe)	Tastes okay initially, but has a recognisable "Nescafe kick" on the back end. The flavor is not very strong.	The fat and sugar in the batter mask much of the coffee flavour.
Specialty Instant Coffee	Better than cheap instant, but still lacking sufficient coffee flavour	Specialty instant often requires a higher dose than commercial instant to achieve the same strength
Well-Brewed Espresso	The best result. Tastes "warm and comforting and friendly". The use of fresh, well-extracted espresso produces a delicious, clear coffee flavour.	Though the additional liquid and acidity from the espresso risked ruining the cake's structure, the resulting cake was far superior.

Structure: The recipe uses a meringue (whipped egg whites and sugar) folded into the batter to achieve a lighter, fluffier sponge texture.
Coffee: Use about 110g of 10% strength espresso to maximise flavour.
Additive (Optional): The speaker incorporated Glycerol Monostearate (GMS), an emulsifier used by commercial bakers, to potentially improve the texture of the finished cake.

For the frosting, the goal was the best coffee flavour possible, not the most coffee flavour, to avoid dominating the excellent sponge.

Buttercream Type: The speaker used a Swiss Meringue Buttercream (based on Stella Parks' recipe) because he finds traditional American buttercream too sweet.
Coffee Infusion: Coffee flavour was added via Coffee Butter, infusing the butter with whole beans.
Method: Infuse 100g of whole coffee beans with 200g of unsalted butter at 90°C for one hour (using a sous vide circulator or a stovetop pot, being careful not to brown the butter).
Whole Beans: Whole beans were used instead of ground to ensure all specks of coffee could be removed for a smooth, particle-free texture in the frosting.
Why Butter? This method extracts different flavour compounds that are fat-soluble rather than water-soluble, adding nuance to the final taste.

The finished, two-tier cake was declared the best coffee cake the speaker had ever had.

Flavour Balance: The coffee flavour was clean and sweet, demonstrating that good coffee was used.
Complexity: The contrast between the oil-infused flavour in the buttercream and the water-extracted espresso flavour in the sponge created a very complete spectrum of flavour.
Recommendation: The speaker recommends using a coffee that is "clean, sweet, and delicious".