Cream butter and sugar with electric mixer until fluffy.
Add eggs, one at a time, beating well after each.
Sift flour, baking soda, and salt into large bowl. On low speed, slowly add flour mixture to bowl, alternating with buttermilk, until combined.
Wrap dough in plastic and chill until firm, about 1 hour or overnight.
Heat oven to 350º. Line 2 baking sheets with parchment paper.
On lightly floured surface, roll chilled dough 1/8 inch thick. Cut out hearts using any 1-3 1/2 inch cookie cutters. Transfer with spatula to baking sheet.
Chill for 30 minutes. Sprinkle with sugar. Bake until just golden. but not too brown, about 10 minutes. Transfer to cookie rack.
To make sandwich hearts, brush bottom heart lightly with jam; cover with second heart.