Put chilled mashed potatoes in bowl. Stir to loosen.
Melt butter and add to the mashed potato. Mix thoroughly and Set aside.
In a separate bowl, mix the egg, milk, scallions, and tarragon together then add to the flour. Mix until smooth, and season to taste. Fold into mashed potato mixture to form pancake batter.
Heat vegetable oil in non-stick frypan on medium-low.
Add the pancake batter into pan, trying to keep them even and on the smaller side.
Cook until golden then flip about one to two minute per side. Add more vegetable oil to pan if needed.
Keep pancakes warm as you make them.
Serve the pancakes along with cured salmon, red onion and capers and sour cream, or try sour cream and apple sauce.