Peel and dice your sweet potato. Place into a sauce pan and fill to cover the potatoes with cold water. Place over medium heat.
Allow the potatoes to cook until fork tender, about 20 minutes.
Remove from heat, drain water.
Measure out about 100g (about ½ cup) of potato into a bowl and mash until smooth. Allow to cool.
Place your butter in the freezer for about 20-30 minutes while your sweet potato cools. Line a baking sheet with parchment.
In a large bowl, add flour, dark brown sugar, salt, baking powder, baking soda, and cardamom. Whisk together.
Take your butter from the freezer and grate into your flour mixture. Halfway through, stop and fluff the butter into the flour using a fork. Grate remaining butter in and repeat.
Create a well in the middle of your ingredients and add cooled sweet potato, heavy cream, beaten egg, and vanilla.
Using a fork, bring the dry ingredients into the wet and mix gently until a rough dough comes together.
Gather the dough together using your hands into a disc like shape in the bowl. Fold the dough over onto itself, press back out and repeat 2 more times.
Place the dough onto your lined baking sheet and press out into a square or circle that is about 1½ inches thick.
Using a knife or bench scraper, cut into 8-9 pieces (depending on shape). When you cut, do not use a sawing motion but cut straight down.
Pull the scones slightly apart, and place into freezer to chill.
Preheat your oven to 425°F.
When oven is ready and scones feel somewhat firm but not frozen (about 20 minutes), remove them from the freezer and brush the tops with heavy cream. If you do not plan to glaze them, I'd recommend sprinkling them with some turbinado sugar.
Bake the scones on the center rack of your oven for about 20-25 minutes. The tops should be lightly golden. Remove from oven and transfer to a cooling rack.
While scones cool, prepare the glaze. In a medium-small bowl, melt your butter and add brown sugar, salt, powdered sugar and milk. Whisk until smooth.
When scones have cooled, spoon or drizzle glaze on top. Store scones in an airtight container for 2-3 days, but they are best right after baking. Enjoy!
