To prepare the elderflower-infused cream, bring the heavy cream to a boil, remove from the heat, and add the elderflowers. Cover and let infuse at room temperature for at least 1 hour, or preferably overnight in the refrigerator. Strain the cream and place it in the refrigerator once it has cooled completely.
To prepare the Breton shortbread, whisk together the egg yolks and sugar until pale and frothy. Stir in the butter until well blended, then combine the flour and baking powder and stir them into the batter. Chill for 2 hours.
Preheat the oven to 325°F (160°C/Gas Mark 3). Roll out the shortbread dough to a diameter that is about ¾ in. (2 cm) greater than that of your pan (which you’ll use later). Place the round of dough on a baking sheet and bake for 15 minutes.
To prepare the strawberry and elderflower mousse, chill the bowl of a stand mixer until very cold. Soak the gelatin in a bowl of cold water until softened. Wash and hull the strawberries and place them in a food processor with the lemon juice and sugar, then process to a coulis.
Heat the coulis in a saucepan, then squeeze excess water from the gelatin and stir it into the hot coulis until dissolved. Let the mixture cool, but do not allow it to set.
Meanwhile, fit the stand mixer with the whisk and pour the infused cream into the well-chilled bowl. Whip the cream until it holds its shape. Using a flexible spatula, gently fold in the strawberry coulis.
To assemble the bavarois, cut the shortbread to fit your pan and place it inside. Pour the mousse over it and chill for several hours, until set.
To serve, carefully remove the bavarois from the pan and decorate with edible flowers and strawberries.