classic sponge with grapefruit curd & my favourite not-too-sweet buttercream icing
baked for a grandma-themed housewarming, June 2023
ingredients
sponge cake
3 eggs, separated
114g caster sugar, halved
1 tsp vanilla paste
90g plain flour
3/4 tsp baking powder
24mL milk (or soy milk)
grapefruit curd
1 tsp gelatine powder
1 egg
1 egg yolk
134g caster sugar
1 grapefruit
85g butter
not-to-sweet buttercream
this icing is a combination of an ermine buttercream, lightened with stabilised whipped cream for a silky, not-to-sweet buttercream that’s sturdy enough for layering
60g caster sugar
16g plain flour
72g milk (or soy milk)
72g butter, softened
1 tsp vanilla paste
150mL thickened cream
1 tsp gelatine powder
method
sponge cake
preheat oven to 180°C
in a medium bowl, whisk the egg whites (3) and half the caster sugar (57g) to medium peaks and set aside
in another bowl, add the egg yolks (3), vanilla (1 tsp) and remaining half of the caster sugar (57g), and whisk until light and fluffy
sift in the flour (90g) and baking powder (3/4 tsp) in batches, gently folding the mixture together in between
add the milk (24mL) and stir to just combine
gently fold in the egg white mixture in batches, mixing until just combined
pour into prepared cake tin and bake for 20-25 minutes
grapefruit curd
in a small cup, sprinkle the gelatine powder (1 tsp) over 1 tablespoon of cold water and set aside for about 5 minutes
zest and juice the grapefruit directly into a small saucepan
add the whole egg (1), egg yolk (1), sugar (134g) to the small saucepan with the zest and juice
add the gelatine and water to the saucepan
whisk over medium-low heat until the sugar and gelatine granules have dissolved
increase the heat to medium-high, whisking constantly until the curd mixture has clearly thickened and coats the back of a spoon
remove from the heat and add the butter (85g)
pour through a fine-mesh strainer to remove the zest and any cooked egg bits, and cover with plastic wrap and set aside
not-to-sweet buttercream
in a small saucepan, whisk together the caster sugar (60g) and flour (16g)
cook over medium heat for 3-4 minutes until slightly toasted-smelling
slowly pour in the milk (72g) and whisk to combine
cook for 4-5 minutes until the mixture forms a thickened roux with a light browned colour
set aside to cool completely
once cooled, add the softened butter (72g) and vanilla (1 tsp) to a large bowl and whisk until pale and fluffy
add in the cooled roux one spoon at a time while whisking and continue until all the roux has been added and the buttercream is homogenous and silky
you could stop here and use the buttercream as-is
in a small pan, sprinkle the gelatine powder (1 tsp) over 1 tablespoon of cold water and set aside for 5 minutes
once the gelatine has hydrated, warm the pan over medium heat until the gelatine has dissolved fully
set aside to cool slightly, while you whisk the thickened cream (150mL) to medium peaks
pour in the gelatine and whisk to combine to make stabilised whipped cream
in a large bowl, fold together the stabilised whipped cream mixture and the buttercream to make your icing
note: due to the gelatine in the curd and the icing, recommend not making these ahead of time as they will set and won’t be able to be piped or spread
assembly
once the cake is out of the oven and has completely cooled, cut it in half and set the top half aside
spread a thick layer of the icing on the bottom, and pipe a thick ring around the outer edge
with a wet spoon, smooth the edges of the icing ring to fill in any gaps. this will keep the curd from leaking out during transport
spoon in the grapefruit curd and replace the top layer of the cake
dust with icing sugar before serving
variations
apple & almond: add a few drops of almond essence both to the sponge mixture and ermine frosting, and replace the grapefruit curd with apple compote
fig & cardamom: add 1-2 tsp ground cardamom to both the cake flour mixture while sifting and roux base, and replace the grapefruit curd with fig jam