Combine flour and baking powder; set aside. In a food processor, turn hazelnuts into meal. Add powdered sugar and blend well. This mixture is called T.P.T (Tant pour tant). Transfer into a pastry bowl. Cream butter and salt and add T.P.T. Add the egg and process until smooth. Scrap down sides of the bowl as it goes. Add half of the flour; mix to combine and add the other half. Wrap up and chill completely. Butter pastry rings generously. Flour work surface and work out half of the chilled pastry to soften – the dough should remain cool and supple. Roll out pastry into a ≈ 0.12''/3mm thick sheet. Cut out into desired diameter and shape. If using perforated tart ring, roll out pastry and finish rolling onto a baking mat to the same thickness (≈3mm); chill to rest or freeze. Chill scraps for later use. Roll out remaining pastry and freeze. Measure out tart sides and cut into 1/4''/6mm strips higher than the ring; keep cool. Moisturize bottom pastry if too dry. Apply half-frozen pastry strips against each side. Lightly prick pastry with a fork. Chill or freeze for about 15 min. Trim off excess pastry and keep frozen. Top tart shells with microwave plastic wrap; fill up with pie weights or rice or beans or cherry pits.
Pre-bake in a 350ºF/180ºC oven for 20 min. Remove weights and bake again for 15 min more; cool to room temp.
Soak gelatin in cold water to soften; and drain. Melt white chocolate and set aside. Heat up a third of the heavy cream, add lavender and leave to infuse for 20 minutes. Sieve and reheat – mix in chocolate. Add gelatin and blend well. Add remaining but cold heavy cream, blend and chill overnight. Whip to medium-firm peaks before using.
Mix all ingredients using an immersion blender, and chill.
Cube 2 white peaches and set aside. Fill half way through the pre-baked tart shell with the vanilla custard. Top with peach cubes; pour more custard to the top. Bake in a 300ºF/150ºC oven for about 30 min or until set. Cool to room temperature; remove rings and chill. Cut remaining peaches into triangle wedges. Whip lavender ganache montée to medium-firm peaks and spread on chilled tart, and arrange peach wedges harmoniously. Enjoy!