Place the tempered salt block on a heavy baking sheet and place on the middle rack of the oven. Turn on the oven to 350F, and heat the block for 1 hour.
In the work bowl of a food processor equipped with a steel blade, combine the flour, sugar, baking powder and salt. Process in short pulses to combine. Add the butter and process in 2 to 3 pulses until it is dispersed evenly.Add the egg yolks and cream and process until the mixture becomes a cohesive dough, about 40 seconds. Add the walnuts and process in pulses until combined, but not so much that the walnuts become finely ground.
Turn the dough out onto a clean work surface and pat into a 9-inch circle about 3/4-inch thick. Cut into 8 wedges. Arrange the cut scones on the hot salt block and bake until puffed and browned, about 18 minutes.
Remove from the salt block with a spatula and let cool on a rack for 5 minutes before serving.