Preheat oven to 180 C / Gas 4. Grease one 20cm round cake tin. Line with greaseproof paper and grease it as well. Lightly flour the tin and paper.
Sift together the flour, 2 tablespoons sugar and salt onto a piece of greaseproof paper.
Place the eggs into a mixing bowl set over a pan of hot (not boiling) water. Add 100g caster sugar to the eggs and whisk until the mixture feels warm (not hot) to the touch, about body temperature. Place the bowl onto a mixer and beat on medium speed until the mixture becomes pale yellow in colour and falls off the end of the whisk attachment in long ribbons. Add the vanilla. Add about 1/3 of the flour mixture to the beaten egg and fold in. Continue to add the remainder of the flour, folding it in each time.
Place about 200g of the cake batter into the bowl containing the melted butter and combine the butter with the batter. Add this back to the main cake batter and fold it in. Pour the batter into the prepared cake tin.
Bake immediately in preheated oven for about 25 minutes or until the top is a light brown. Cool the cake in the tin on a wire cooling rack for about 10 minutes; then turn out onto the rack. It is best to invert it again onto another rack so that it cools with the top up.
Let cool completely before slicing, filling or icing.