1. Cut the kabocha squash into fourths and scoop out the seeds. Reserve one quarter wedge and use the other three for a different dish. 

2. Prepare a pot with a steamer basket. Pour a couple of inches of water into the pot and bring to a boil. Once the water comes to a boil, use tongs to carefully put the quarter wedge of kabocha squash inside the steamer basket. Place the lid on top and steam for 10 minutes. 

3. After 10 minutes, remove the steamed kabocha squash from the steamer basket using the tongs. Let cool on a plate or tray. When it’s cooled enough to handle, cut into large pieces, about 3/4 inch or 2 centimeter cubes. Lay the cut pieces in a single layer, cool completely, and set aside until ready to use. 

4. In a large bowl, sift the flours, sugar, salt, and baking powder together. Add the cold butter and cut in with a fork, pastry cutter, or bench scraper. Once the mixture looks like wet sand (a few uneven chunks of butter is ok), add the buttermilk and about 2/3 of the diced kabocha squash. Stir until roughly incorporated

5. Pour the dough onto a clean surface. Carefully work the dough into a rough ball, and then flatten into an 8 inch disc. Using the bench scraper or a sharp knife, cut into 8 triangles. Place the triangles onto a parchment-lined tray or baking sheet and freeze for 2 hours, overnight, or up to 3 months. If freezing for a long period of time, place frozen scones into an airtight bag or container. 

6. When ready to bake, place the oven rack to the top third position and preheat to 375 degrees F. 

7. Get two baking sheets and stack on top of each other (this will help prevent the bottoms from burning). Line with parchment paper. Prepare the egg wash by whisking the yolk and buttermilk together. Remove the scones from the freezer and arrange on the baking sheet, leaving about 3 inches of space in between each scone. Brush with the egg wash and sprinkle with a  generous pinch or two of turbinado sugar. 

7. Bake for 30 minutes or until tops and bottoms are golden. Remove from the oven and immediately transfer to a cooling rack. Scones are best eaten the day they’re made, however, they can be kept in an airtight container at room temperature for 1 day or in the refrigerator for 5 days. Serve by reheating in a preheated 350 degrees F oven for about 10 minutes.