There now seem to be two forms, both quite commonly made. One with a solid paste top is cut into squares and served as a cookie, one with a latticed top is served as a plated desert.
Compare with: Cumberland Currant Cake

Cumberland Rum Nicky
Image: www.booths.co.uk
The origin of the name is obscure, but the African and Caribbean ingredients originate from the late 18th Century when the Cumbrian ports of Whitehaven, Workington and Maryport were centres of the notorious 'triangular trade' - shipping sugar, rum and ginger in from the Caribbean, taking English cloth to West Africa and slaves back to North America.
Original Receipt from lake-district-guides.co.uk;CUMBERLAND RUM NICKY
Ingredients:
6 ozs (170g) rich short pastry, 1 oz (28g) preserved ginger, ½ oz (14g) caster sugar, 2 oz (54g) chopped dates, 1 oz (28g) butter, 1 tablespoon rum
Method:
Line a buttered pie plate with half the pastry and cover with the dates and ginger.
Beat the butter, sugar and rum and pour over the filling.
Cover with the remaining pastry and bake in a moderately hot oven for 10-15 minutes.
Then turn down the heat and bake for approximately 30 minutes.
Serve with creme fraiche.