In mixing bowl, cream butter and sugar until fluffy, about 4 minutes.
Add eggs, one at a time, beating well after each.
On low speed, gradually add flour, baking soda and salt, alternating with buttermilk, until combined.
Wrap dough in plastic; chill until firm. You can do this 1 hour or overnight.
Heat oven to 350° with two racks centered.
On lightly floured surface, roll chilled dough 1/8 inch thick. Cut out hearts using any 1- to 3 1/2-inch heart cookie cutters. If desired, cut centers out of some hearts, which is what you will need to do for the jam to show.
Transfer cookies to cookie sheets lined with parchment paper, using a spatula.
If you have time, Chill for 30 minutes on cookie sheets. I was short on time so I left out this step and just cooked the cookies less time, and it worked out fine.
Sprinkle cut cookies with sanding sugar, if using.
Bake until just golden but not too brown, about 10 minutes. Transfer cookies to rack.
*To make sandwich hearts: Brush bottom heart lightly with jam; cover with a second heart with center cut out; jam will adhere hearts. Fill cut-out area with more jam if necessary.