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1
Beat the softened butter and caster sugar together until the mixture is pale and holds a ribbon, then add the egg yolks one at a time, beating well after each addition so the fat absorbs each yolk fully before the next arrives. Add the lemon zest and Trentino marc Grappa and mix until fully incorporated, then fold in the semolina in two additions — the batter will be thick. Cover the bowl and leave it to rest at room temperature for 30 minutes so the semolina drinks in the moisture and swells; it is ready when it has visibly puffed and the batter no longer looks sandy.
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2
This is where the dish lives or dies. In a clean bowl, beat the egg whites to firm peaks — they must hold a straight, upright point when the whisk is lifted and show no slump, because it is this structure alone that will lift a batter too heavy to rise any other way. Fold the whites into the rested semolina batter in three additions, working from the bottom of the bowl outward, sacrificing no more volume than is necessary; the finished batter should be aerated and just hold its shape, for a heavy, deflated mass means the leavening is lost and the cake cannot recover.
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3
Heat the oven to 170 °C, then butter and line a 22 cm round cake tin. Pour the batter in, level the surface, and set the tin in a deep roasting tray; pour boiling water into the tray to come 3 cm up the sides of the tin so the gentle, indirect heat keeps the dense semolina crumb moist rather than tight and dry. Bake in the water bath for 30 minutes — the surface should be set and beginning to colour, the edges just pulling from the tin.
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4
Lift the cake tin from the water bath and return it directly to the oven shelf so the base and sides can now take direct heat and firm without the steam softening the crust further. Bake for a further 20–25 minutes until the centre is just set — a skewer inserted at the midpoint should emerge with only the faintest moist crumb. Do not wait for a clean skewer: a fully dry centre means the semolina has over-dried and the characteristic moist, dense crumb is gone.
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5
Cool the cake in its tin for 15 minutes before unmoulding onto a rack so the crumb has time to settle and will not tear when turned out. Allow to cool completely, then dust with icing sugar and lay a curl of lemon zest at the centre. The cake is ready to serve now, but continues to improve over the following days as the Grappa permeates the crumb.