Cut the butter into small cubes and place into freezer for 1 hour.
Adjust the oven rack to the center position and preheat your oven to 400°F. Line one baking sheet with parchment paper.
In a large bowl, combine flour, sugar, salt and baking powder.
Add the butter and use a pastry cutter, fork or your hands to mix it with the dry ingredients.
Add the egg and mix until combined.
In a small bowl, mix sour cream with heavy cream. Add ⅔ of it to your butter mixture. Mix until incorporated and add more if dough is still too crumbly.
Add white chocolate and frozen blackberries.
Place the dough into a floured surface and divide into 2 equal parts. Lightly knead each into a disc, at least 1 inch tall. Flip the disc once, then cut it into 8 pieces.
Place the scones in the prepared baking sheet, each spaced about 2 inches apart.
Bake for about 15 minutes on 400°F, or until golden brown on top.
Remove from the oven and let it cool for 20 minutes before pouring the glaze on top.
For the glaze, melt the butter.
Mix the melted butter with the lime juice (and zest if using).
Add the confectioner sugar, mixing until homogeneous. If the glaze is too runny, add more sugar, one tablespoon at a time.
Pour over the scones as desired and let it set before serving.