Sift together the flour, salt and baking powder onto a piece of parchment paper. Transfer it to the bowl of a food processor fit with the knife blade. Add all but 1 tablespoon of sugar, pulse once or twice, then add the butter and pulse until the mixture is about the size of coarse cornmeal.
Add 1 cup of cream (250ml) all at once, and lightly . mix until the dough gathers together but doesn't completely adhere. Be very careful not to over mix. If you are using a food processor, pulse just two or three times.
Turn the dough out onto a well-floured surface, dust it lightly with flour, and press it down with a rolling pin until it is somewhat rectangular in shape and about 1/2-inch (1.25cm) thick. Fold it in thirds like a business letter, and turn it one quarter turn (90 degrees). Lightly flour it and gently press it out again until it is an even rectangle, about 1/2-inch (1.25ml) thick, fold it in thirds, and turn it another quarter turn.
Press the dough into a 9-inch (22.5cm) circle on a piece of aluminum foil. Cut the dough into 8 equal-sized pieces. Sprinkle the remaining tablespoon of sugar in an even layer on top of the dough.
Preheat the gas grill with the plancha to 425F (220C).
When the grill and the plancha are hot, transfer the dough on the aluminum foil to the plancha, cover the grill and bake the shortbread until it is puffed and golden, which will take 12 to 18 minutes. The baking time depends on the grill you are using; check the shortcakes after 12 minutes.
When the shortcake is baked, transfer it immediately to a wire rack to cool.
Using a chilled whisk or electric mixer attachment and bowl, whip the remaining 1 cup (250ml) of cream with the vanilla and the confectioners' sugar until it holds soft peaks.
To serve, divide the shortcake among 8 plates. Top each with an equal amount of the berries and a big dollop of whipped cream. Garnish with the herb leaves if desired, and serve.