Preheat the oven to 375 degrees F. Line a baking sheet with parchment paper.In a large mixing bowl, combine the flour, baking powder, sugar, and salt. Add the butter and toss with the flour. Add the buttermilk and egg. Mix until just combined, being careful not to over-mix. Fold in the blackberries and white chocolate chips. It's OK if some of the berries burst. Turn the dough out onto a floured surface. Pat into a 1-inch thick square. Now cut the dough into 9 large squares or 12 smaller squares. Place on the prepared baking sheet. Brush the tops of the scones with cream. Transfer to the oven and bake until golden brown, 20-22 minutes. Meanwhile, make the lavender glaze. Warm the milk and butter in a small saucepan set over medium heat until steaming, about 3-5 minutes. Remove from the heat and stir in the lavender. Cover and steep 5 minutes. Strain the milk through a fine-mesh strainer and discard the lavender. Stir in the vanilla and powdered sugar, adding more to your taste. Serve the scones warm, drizzle with the glaze and smeared with butter. Enjoy!