In a small saucepan, heat oil over medium-low heat. Add shallot; cook until fragrant, approximately 2 minutes.
In a small bowl, whisk together jam, cornstarch, salt, and pepper. Add jam mixture to shallot in saucepan, and bring to a boil. Cook, stirring constantly, until thickened, 2 to 3 minutes. Transfer mixture to a small heatproof bowl, and let cool completely.
Fold chives into jam mixture. Transfer mixture to an airtight container, and refrigerate until ready to use, up to 3 days.
Shingle half of chicken slices onto 6 bread slices. Top chicken with jam mixture, spreading in an even layer. Shingle remaining chicken on top of jam layer. Cover with remaining 6 bread slices.
Using a long, serrated knife in a gentle sawing motion, trim and discard crusts from bread. Cut each sandwich into 4 (approximately 3×1-inch) rectangles. Cover with a damp paper towel, place in an airtight container, and refrigerate until serving time.
Garnish with chives, if desired.
Notes
*We used Stonewall Kitchen Black Raspberry Jam.
Recipe by TeaTime Magazine at https://teatimemagazine.com/chicken-and-black-raspberry-tea-sandwiches/