Put sugar and water in a small saucepan, bring to a boil over medium heat, stirring constantly to dissolve the sugar before it boils, then simmer until the sugar syrup reaches 110 °C.
Add lemon juice, stir briefly, and continue to simmer (stop stirring) until the syrup reaches 115°C. Immediately remove it from the heat and let it cool down to about 40 °C in a cold water bath.
Whip the cooled transparent sugar syrup with kneading picks to a snow-white, firm sugar glaze
Cut the shortcrust pastry base into two equal parts, spread one part with the jam and carefully place the second part on top.
Heat the glaze in a water bath until it thickens.
Put about 1 tablespoon of the glaze in a small mixing bowl, add cocoa powder and stir. (If the chocolate glaze is too dry, you can add a few drops of water. Don't overdo it with the water, though).
Pour the remaining white glaze over the "shortcake" and paint thin stripes on it from the chocolate glaze.
Let the "cake" rest for about 2 hours at room temperature and cut it into 10 equal-sized slices.