Line a baking sheet with parchment paper. Set aside.
In a large bowl, whisk together the flour, baking powder, 3 tbsp sugar, and salt to combine.
Add the butter and toss with dry ingredients until coated. Work the butter into the dry ingredients by using your fingertips and pinching it until there are lots of flat shards and pea-sized bits remaining. Those small, uneven pieces of butter create steam pockets as the shortcakes bake, contributing to a tender, flaky end result.
Create a well in the center of the dry ingredients. Add 1 cup heavy cream into the well. Working with a fork or rubber spatula, mix until large, shaggy clumps form.
Knead once or twice (don’t over mix) until it forms a mass and most of the flour has been incorporated into the dough.
Transfer the dough to a lightly floured surface. Pat to a 1 inch-thick square (it should be about 6x6").
Cut the dough into 4 equal squares. Stack them all on top of each other, creating a tall tower of dough. Press down on the stack with your hands, flattening the dough back to a square. This stacking method is what will create lots of flaky layers.
Pat to a rectangle about 1" thick. Using a round cookie cutter, cut into biscuits or cut the rectangle in half lengthwise, then cut each of those 2 rectangles into 3 squares. Transfer to the prepared baking sheet. Freeze for about 15 minutes or 30 minutes in the fridge. (This will help ensure that the shortcakes keep their shape in the oven.)
Meanwhile, preheat the oven to 400°F.
Brush tops with remaining heavy cream and sprinkle with remaining sugar.
Bake shortcakes until golden brown all over, 18-25 minutes.
Let shortcakes cool for at least 15 minutes. Gently split shortcakes in half with your hands or using a knife. Place bottom halves in serving bowls. Divide whipped cream and strawberry jam among bottom halves. Drizzle any syrupy juices left over. Close with top halves of shortcake. Serve.