• Adjust an oven rack to the middle position and preheat the oven to 400F [200C]. Stack two sheet pans on top of each other and line the top sheet with parchment paper (this helps keep the bottoms of the scones from browning too quickly).
• In a large bowl, combine the sugar and lemon zest with your hands, rubbing the zest into the sugar. Add the flour, baking powder and salt, and whisk to combine.
• In a medium bowl or liquid measuring cup, whisk together the sour cream or creme fraiche, lemon juice, egg, egg yolk, and vanilla.
• Add the butter to the dry ingredients and use a pastry cutter to cut in the butter until the flour-coated pieces are the size of peas. Add the wet ingredients and fold with a spatula until just combined. Add the white chocolate, gently folding it into the dough.
• Transfer the dough to a lightly floured surface and knead four to six times, until it comes together, adding more flour as necessary if the dough is sticky. Pat the dough gently into a square and roll into a 12 in [30.5 cm] square (again, dusting with flour as necessary). Fold the dough in thirds, similar to a business letter. Fold the short ends of the dough in thirds again, making a square. Transfer it to a floured sheet pan or plate and place in the freezer for 10 minutes.
• Return the dough to the floured surface, roll it into a 12 in [30.5 cm] square, and fold it thirds, Place the dough seam-side down and gently roll the dough into a 12 by 4 in [30.5 by 10 cm] rectangle. With a sharp knife, cut it crosswise into four equal rectangles, then cut each rectangle diagonally into two triangles. Transfer the triangles to the prepared sheet pan.
• Brush the tops the triangles with a little heavy cream, making sure it doesn't drip down the sides, and sprinkle the tops generously with sugar. Bake for 18 to 25 minutes, rotating the pan halfway through, until the tops and bottoms are light golden brown. Transfer the sheet pan to wire rack and let cool for 5 minutes before glazing.

For the glaze

• While the scones are baking, in a medium bowl, whisk together the confectioners' sugar, 2 tablespoons of the lemon juice, and the melted butter until smooth. Add more juice, 1 tablespoon at a time, to thin the glaze to your preferred consistency.
• Pour the glaze over the tops of the scones and use the back of a spoon to smooth the tops. Let set before serving. Scones are best eaten the same day they are made.