Heat oven to 400°F. Rub chicken with oil, 1/4 teaspoon salt and pepper, then roast in a small baking dish for 22 to 28 minutes until the thickest part reaches 165°F.
Toast almonds on a small pan in the same oven for 4 to 6 minutes, watching closely. Rest chicken for 5 minutes, cut into small pieces and transfer to a shallow container; refrigerate promptly for 40 minutes until cold.
Crush the cooled almonds lightly. Stir sour cream, mayonnaise, Dijon and remaining salt together, then fold in the cold chicken, grapes, celery and almonds.
Butter one face of each bread slice. Spread the salad equally over six slices in a flat layer reaching the edges, then close with the other six slices, buttered faces inward.
Trim crusts and cut each sandwich diagonally in both directions to make four small triangles. Keep the 24 triangles covered and refrigerated; serve two per portion within 24 hours.