Place a rack in center of oven; preheat to 400 degrees Fahrenheit. Line a rimmed baking sheet with parchment paper.
Whisk 2 cups flour, 1 ½ tsp. baking powder, ¼ tsp. baking soda, 4 tsp. sugar and 1 ½ tsp. salt in a large bowl to combine.
Finely grate zest of about half of 1 lemon right into bowl (reserve lemon for another use).
Cut 1 stick cold butter into ½ inch pieces and toss with dry ingredients until coated. Work butter into dry ingredients by pinching and rubbing it between your palms until there are lots of flat shards and pea-sized bits remaining. Those small, uneven pieces of unincorporated butter create steam pockets as the shortcakes bake, resulting in a tender, flakey end result.
Combine ¾ cup sour cream and 1/3 cup heavy cream in a medium bowl. Create a well in the center of the dry ingredients. Scrape cream mixture into well. Working in circles from inside the well outwards with a rubber spatula, mix dough until large, shaggy clumps form.
Transfer to a lightly floured surface. Pat to a 1 inch-thick square (it should be about 6 in x 6 in).
Cut dough into 4 equal squares, then stack them all on top of each other, creating one very tall tower of dough. Press down on stack with your hands, flattening dough back to a square. This stacking method will create lots of flaky layers. Pat to a 7.5 inch x 5 inch rectangle about 1 inch thick.
Cut rectangle in half lengthwise, then cut each of those 2 rectangles into 3-4 squares. Transfer squares to prepared baking sheet. Sprinkle tops with 1 Tbsp. sugar. Freeze 10 minutes. (This will help ensure that the shortcakes rise – rather than spread out – in the oven.)
Bake shortcakes until golden brown all over, 20-25 minutes.
Let shortcakes cool at least 10 minutes. Gently split shortcakes in half with your hands. Place bottom halves in serving bowls. Layer jam onto bottom halves and close with top halves. Enjoy!