Heat sugar, lemon juice, zest, and 45ml water in a pan until sugar melts.
Add strawberries and stir gently into the syrup.
Boil the mixture for 4 to 10 minutes depending on desired thickness.
Verify the jam has reached 105°C using a thermometer.
Remove from heat, cool slightly, and skim off any froth.
Transfer jam to a sterile jar and seal.
Preheat the oven to 190°C.
Grease and flour two 20cm sandwich tins.
Cream the butter, sugar, and vanilla together until smooth.
Beat in the eggs one at a time.
Fold in the sifted flour and divide the batter between tins.
Bake for 20 to 25 minutes until golden and springy.
Turn cakes out onto a wire rack to cool completely.
Spread one sponge with jam and a layer of whipped cream.
Top with fresh strawberries and the second sponge.
Dust the top with icing sugar to finish.