Preheat oven to 340°F.

Grease and line the base of an 8‑inch round tin with parchment; lightly flour the sides.

In a large bowl, rub the lemon zest into the sugar until fragrant and slightly damp. This releases the lemon oils and distributes the flavor throughout the crumb.

Beat in the butter until pale and fluffy, about 2–3 minutes.

Beat in the eggs one at a time, scraping down the bowl after each addition.

Beat in the mascarpone, followed by the lemon juice and vanilla. The batter will look loose at this stage; that’s exactly right.

In a separate bowl, whisk together the flour, baking powder, and salt. Fold into the batter just until smooth, taking care not to overmix.

Scrape the batter into the prepared tin and smooth the top.

Bake for 35–40 minutes, until lightly golden and a skewer inserted into the centre comes out clean.

Cool in the tin for 15 minutes, then turn out onto a rack.

Combine the limoncello, lemon juice, and sugar in a small saucepan.

Heat gently, stirring, just until the sugar dissolves. Do not boil.

While the cake is still warm, brush the syrup generously over the surface, allowing it to soak in gradually.

Let the cake cool completely.

Add the mascarpone to a mixing bowl and beat briefly to loosen. Beat in the icing sugar, lemon zest, and vanilla.

Add the cream and whip on medium speed until thick, smooth, and softly billowy. Stop as soon as it holds soft peaks.

Assembly

Spread the mascarpone cream over the cooled cake, leaving a slight rim around the edge.

Spoon the lemon curd over the cream and gently swirl with the back of a spoon or a table knife.

Chill for 20–30 minutes before slicing for the cleanest cuts.