1. To make the biscuits, preheat the oven to 400ºF (200ºC). Line a baking sheet with parchment paper or a silicone baking mat.
2. Sift the flour, baking powder, salt, and sugar into a bowl. Using a pastry blender, cut in the butter until it’s in pieces the size of corn kernels. (You can also use a food processor or stand mixer with the paddle attachment.) Add the cream or buttermilk and mix until just blended.
3. On a lightly floured countertop, briefly knead the dough just until it comes together. Do not overwork the dough; it’s better to work it less and have rustic-looking biscuits than ones that are tough.
4. Roll the dough until it’s 3/4-inch (2 cm) thick and with a 3-inch (10 cm) biscuit cutter, cut out six individual biscuits, dipping the cutter in flour between cutting each biscuit. You can gather the scraps and re-roll to cut out a few more biscuits. Put the biscuits on the baking sheet. For the glaze, mix the egg yolks with the cream or milk.
5. Brush just the tops of the biscuits with the glaze and bake for 15 to 20 minutes, until they’re browned on top and up the sides. Remove from oven and let cool.
6. Make the peach coulis by mixing the diced peaches with 2 tablespoons of sugar and kirsch, if using, with your hands, squeezing firmly the diced fruit to mash it up and help it release its juices. Let stand at least an hour.
7. Whip the cream until it begins to get stiff, then whip in the sugar and vanilla extract and continue to whip until it holds its shape.
8. Slice the peaches and toss them in a bowl with a sprinkling of sugar.
9. To assemble the shortcakes, cut each biscuit in half and place the bottoms on six plates. Divide the peach coulis amongst the biscuits, pressing it in a bit with a spoon to encourage the juices to saturate the biscuit. Add a few peach slices on top of each, then add a generous dollop of cream. Finish with the rest of the peach slices and top with the crown of the biscuit. Drizzle with warm butterscotch sauce.