Preheat oven to 350 degrees. Line two large baking sheets with parchment.
Vanilla batter: in a large bowl, beat the butter and sugar until light and fluffy, about 2-3 minutes, then whip in the vanilla. Add the flour and salt, mixing until a soft cohesive dough forms, scraping down the sides of the bowl as needed. If the dough seems too tough, add 1/2 tbsp water. I did not have this issue.
Chocolate batter: in a separate large bowl (or the same one, cleaned out), beat the butter and sugar for 2-3 minutes, then whip in the vanilla. Add the flour, cocoa powder, and salt, mixing until a soft cohesive dough forms, scraping down the sides of the bowl as needed. The chocolate dough will be a bit tighter than the vanilla. Again, add 1/2 tbsp water to the dough and mix if it's not sticking.
Flour a clean, flat surface. Divide the vanilla dough in half and roll into a 1/4 inch thick rectangle. Using a 2 inch biscuit cutter, gently press rounds out of the dough and set onto the prepared baking sheet. Leave about 1/2 inch space between each round - they don't spread much.
Gather and re-roll the dough as needed to cut more rounds. Repeat with the remaining vanilla dough and the chocolate dough. I got 20 rounds each out of the vanilla and chocolate doughs.
Set the baking sheets in the oven and bake for 12-15 minutes until the edges are set. Cool on the baking sheets.
When the cookies are cooled, make the buttercream. Whip together the butter, powdered sugar, jam, 1 tablespoon heavy cream, and vanilla until soft and fluffy, about 4-5 minutes. This buttercream should be a bit stiffer than what you'd prefer when frosting a cake, but if it's too gritty, add 1 tablespoon additional heavy cream.
Scoop the frosting into a piping bag or a large ziptop bag with a corner snipped out. Lay 20 of the cookie rounds on a flat surface - mix and match between the vanilla and chocolate!
Pipe the frosting into the center of each cookie and top with the opposite cookie as the bottom layer. Serve immediately, or store in the fridge.