Drain cherries, reserving all the juice. Melt butter in a saute pan. Stir in cornstarch and sugar until smooth. Gradually stir in the reserved cherry juice. The mixture will thicken up. When it boils, pour in the brandy and turn off the heat. Light the brandy in the pan with a lighter or long match. When the flame dies down, stir in the cherries and serve over ice cream.