For the cake and filling:
1 3/4 cups (8 ounces) all-purpose flour, plus more for dusting
2 1/2 teaspoons baking powder
3/4 teaspoon salt
3 large eggs, at room temperature
Finely grated zest and juice of 1 lemon (about 4 tablespoons)
1 1/2 sticks (6 ounces) unsalted butter or margarine, at room temperature, plus more for greasing
3/4 cup plus 2 tablespoons (6 ounces) superfine sugar
27 jumbo marshmallows (about 7 ounces)
Lemon curd for filling, store-bought or homemade
For the icing:
2 cups (8 ounces) powdered sugar
Preheat the oven to 350°F with a rack positioned in the middle of the oven. Grease and flour two 8-inch cake pans (you can also use 9-inch cake pans, see the testing notes above).
Sift together the flour, baking powder, and salt in a bowl. Beat the eggs and 1 tablespoon of the lemon juice in a small bowl (set the rest of the lemon juice aside for the icing). Set both bowls aside.
Cream the butter, sugar, and lemon zest in a stand mixer or with a hand mixer until light. Add 1/4 of the egg mixture and one spoonful of the flour mixture to butter-sugar mixture, and beat well. Add the remaining egg mixture in a few additions, each with a spoonful of flour, and beat well. You will not use all of the flour mixture.
Using a spatula, fold the remaining flour into the bowl and mix well, but do not beat too hard. Divide the mixture between the 2 pans and spread into an even layer.
Bake for 20 to 30 minutes, or until the cake pulls away from the sides of the tins. Then remove from the oven and let cool 15 minutes (keep the oven on). Remove the cakes from the pans and onto a wire rack.
Line a baking sheet with parchment paper and place one of the cakes on it (the cake can still be warm). Cut the marshmallows in half with scissors, then place the marshmallows sticky-side down in a single layer on top of the cake, arranging them as close together as possible.
Place the marshmallow-covered cake back in the oven for a few minutes so the marshmallows melt just a little, about 5 minutes. Remove from the oven and let cool completely.
To assemble, spread a generous layer of lemon curd over the second round of cake (the one not covered in marshmallow). Set the layer with the melted marshmallows on top of this cake layer so the lemon curd is sandwiched between and the melted marshmallows are on top of the cake.
To make the icing, sift the powdered sugar into a saucepan, add 2 tablespoons of the remaining lemon juice and mix into a smooth paste. Warm gently over a low heat, beating well until it is of pouring consistency. You might have to add a little lemon juice, water, or milk, but make sure it is ve