Preheat oven to 325ºF. Grease and flour two 9x2” round cake pans and line with parchment circles.
Add butter, cream cheese, oil, sugar and lemon zest to bowl of standing mixer fitted with the paddle attachment. Beat on medium-high speed until very light and fluffy, about 4 minutes. Pause a couple of times to scrape down mixing bowl.
Beat in eggs and vanilla extract.
Add the buttermilk and beat on low speed to just combine.
In a separate bowl, whisk together flour, baking soda, baking powder and salt. With mixer speed on low, beat in half the flour mixture until just combined.
Add remaining flour mixture and continue beating on low only until flour is incorporated, pausing once to scrape down mixer bowl and paddle. Watch closely and do not overmix (this keeps the cake soft).
Spoon the cake batter evenly into prepared pans. I find it easily done with an ice cream scoop. Smooth out the top of the batter and place in the centre of preheated oven.
Bake about 35-40 mins, until it reaches 190ºF with a meat thermometer or a tester comes out clean.
Let the cake cool in pan for 5 minutes, then run a knife around the edges, place a wire rack on top, and use oven mitts to flip the whole thing over. If it doesn’t release easily, place a hot towel on top to steam the bottom out. Let cake cool completely.
To make lemon curd filling:
Bring 1" of water to a strong simmer in a small saucepan.
Whisk together sugar, lemon juice and zest to a heat-safe bowl and place it over a pot of simmering water (or use a double boiler). Whisk in the egg yolks.
Whisk constantly until it thickens to the consistency of yogurt. It'll take 6-10 minute.
Remove from heat and whisk in the butter. Strain through a fine-mesh sieve if there are any lumps. Refrigerate until cool.
To make frosting:
Bring 1" of water to a strong simmer in a small saucepan. Combine sugar, egg whites and lemon juice in a large heat-proof bowl set over top (or in a double boiler). Make sure your bowl does not touch the water.
With burner heat on low, beat with hand mixer until glossy, soft-medium peaks form, 7 minutes. Remove bowl from heat and beat in vanilla extract. Let cool to room temperature.
To assemble:
Place one cake layer on your serving plate. Pipe or spoon a ring of frosting around the border to make a dam that prevents the filling from spilling out. Spoon the lemon curd to the middle and spread it out.
Place the second cake layer on top. Spread remaining frosting over the top and sides of the cake.
Video
Notes
7-minute Frosting:
You may substitute 3/4 tsp cream of tartar for the lemon juice.
This soft, meringue-like frosting is so fluffy and white. I’ve been testing this recipe for years to make it foolproof for you. It won't go hard if you don’t over-beat it.
Beat for 7 minutes on high speed of a handheld mixer. You’ll have soft-medium peaks at this point.
Don’t try to push it further to stiff peaks. You’ll dry out the meringue as steam escapes and the frosting will form a crust as it dries. It also produces less frosting as you whip the moisture out.
You can toast the frosting using a handheld kitchen torch for a lemon meringue pie look.
This yields about 5 1/2 cups of frosting, enough frosting to frost and fill a 2-layer 9” cake.
It stays soft and fluffy for a couple of days at room temperature, but I recommend this one for enjoying on the day you make it.
Lemon Curd:
YIELDS 1 ½ cups lemon curd.
Use a non-reactive metal bowl (not copper). Do not use glass, as the glass may shatter.
Use a microplane to grate the lemon zest. This ensures that the zest is fine enough to disappear into the curd without needing to strain it out later.
Whisk constantly while cooking to avoid curdling the eggs. The water should only be maintained at a simmer, not a boil, and the water should never touch the bottom of the bowl.
The lemon curd will continue to thicken as it chills and sets.
Add the sugar, lemon juice, and zest first, whisk it, then add the egg yolks. If you add the yolks to the double boiler first, they will likely start cooking (curdling) before adding the other ingredients.