While the butter is freezing, combine the flour, baking powder, sugar and salt in a large mixing bowl. You’ll zest the lemon right into the dry ingredients.
In a large glass measuring cup, whisk together egg, heavy cream and vanilla. Store in the refrigerator until needed.
Remove frozen butter from the freezer and use your cheese grater to grate the frozen butter directly into the dry ingredients. Use a pastry cutter or fork to work the butter into the flour mixture. Pour the heavy cream and eggs over the dry ingredients and mix until just combined.
Add the mini white chocolate chips to the dough. Gently fold the chips into the dough. It's okay if they are not well combined.
Turn the dough out onto a floured surface and fold the dough over itself and then flatten it to incorporate the chips. Do this 4-5 times to be sure that the chips are evenly distributed throughout the dough. You want the dough to come together and look uniform with flecks of butter throughout. Form the dough into a 8-10 inch circle and use a bench scraper or sharp knife to cut the dough into 8 triangles.
Arrange on a quarter sheet pan and place scones into the freezer for 15-20 minutes before preheating the oven to 400°. After the 20 minutes, arrange the chilled scones on a parchment lined, half sheet pan. Brush the scones with heavy cream and bake at 400° for 18-22 minutes or until golden brown.
While the scones are baking, make the lemon glaze.