Coffee and Walnut Cake with Espresso Buttercream

Coffee and walnut cake is the quiet hero of the British tea room, the one that gets overlooked in the display case in favour of flashier things and then turns out to be the best cake there. It is not showy. It is a soft, buttery sponge the colour of milky coffee, studded with toasted walnuts and layered with coffee buttercream, and it has been winning people over on cake stands and church fête tables for the better part of a century. Its greatness is entirely in the balance: bitter coffee against sweet buttercream, soft crumb against crunchy nut.

A very British kind of American coffee cake

The pairing of coffee and walnuts in a cake is older than you might expect and arrived in Britain from America, where “coffee cake” once meant a cake flavoured with coffee. The idea took particularly firm root here in the mid-twentieth century, helped along by the spread of instant coffee, which gave home bakers a concentrated, reliable coffee flavour without the faff of brewing and reducing. That is why instant remains the classic choice: a spoon of good instant espresso powder dissolved in a little boiling water delivers a punchier, more even coffee hit than filter coffee ever could, because it is not diluted by all that water.

Walnuts are the traditional partner for a reason that goes beyond habit. Their flavour is faintly bitter and tannic, with a resinous depth that stands up to coffee where a sweeter nut would be swamped, and toasting them first turns that bitterness into something warm and buttery. It belongs to the same cosy tea-room family as a good sticky ginger cake, all built for slicing thickly and eating with a strong brew. My one addition is a spoon of tahini in the buttercream, which deepens the whole thing without anyone quite being able to say why.

Toast the walnuts, always

The single most common mistake with this cake is using walnuts straight from the bag. Raw walnuts are pale and can carry a faint stale or soapy edge, because their high oil content oxidises quickly once shelled. A few minutes in a hot oven transforms them: the heat drives off surface moisture, deepens the flavour through gentle toasting and crisps the texture so the nuts hold a little crunch in the finished cake rather than turning soft and damp. Let them cool completely before chopping, both so they chop cleanly and so they do not melt the butter when they meet the batter. It is five minutes of work that lifts the cake from pleasant to properly good.

The tahini in the buttercream

Coffee buttercream is lovely on its own, but it can veer towards flat sweetness if the coffee is not strong. My fix is two spoons of tahini, the smooth sesame paste, beaten in with the espresso. Sesame and coffee share a family of roasted, nutty, slightly bitter flavours, and the tahini amplifies the coffee while adding a gentle savoury richness and a touch of grown-up bitterness that keeps the icing from being sugary. It also loosens the buttercream very slightly, giving it a silkier, more spreadable texture. Use light, well-stirred tahini rather than a dark, bitter one, and taste as you go; you want the sesame to hide behind the coffee, felt more than named.

Get the buttercream light by beating the butter alone for a good five minutes before any sugar goes in. This whips air into the fat and warms it just enough to take the icing sugar smoothly, so you end up with a pale, mousse-like cream rather than a dense, sweet paste. Make sure the espresso is fully cool before it goes in, or it will melt the butter and slacken the whole thing.

Getting clean layers

This is a robust, everyday cake rather than a fussy one, but two small things lift the finish. Level the batter well before it bakes, since coffee sponge domes readily thanks to all that beaten butter, and a domed layer makes for a wobbly stack. And let the sponges cool completely before you go anywhere near them with buttercream: a warm crumb melts the fat, the icing slides, and the walnuts on top slowly slump into the puddle. Patience of half an hour is the whole trick. If you are in a hurry, a spell in the fridge sets the sponges fast and firms them for a cleaner coat.

Storage and making ahead

The sponges can be baked a day ahead and kept wrapped at room temperature, and the buttercream can be made a day ahead too, kept covered in the fridge and beaten briefly to loosen before use. Once assembled, the cake keeps in a tin for three or four days and, like most butter-rich sponges, is arguably better on day two once the crumb has settled and the coffee has mellowed into it. Keep it somewhere cool but bring slices to room temperature before serving, since cold flattens both the coffee and the sesame. The unfilled sponges freeze well for up to three months.

Variations

Coffee and walnut takes happily to small changes. Swap the walnuts for toasted pecans for a sweeter, more buttery nut, or fold a handful of chopped dark chocolate into the batter for a mocha version. A tablespoon of coffee liqueur brushed over the warm sponges turns it into a grown-up celebration cake, echoing the boozy depth of a chocolate Guinness cake. For a lighter finish, skip the full buttercream coat and simply sandwich the layers, dust the top with icing sugar and press walnut halves into a thin layer of icing, which is closer to how your grandmother’s version probably looked.