Gem cakes are light, spicy, small cakes made in New Zealand, traditionally in a small loaf shape.
Making them small helps speed the baking time. From start of opening recipe book to mixing to baking to table, you only need about 30 minutes.
They are served warm with butter, or with jam and cream, or with lemon curd.
Ginger flavoured ones is a classically popular choice; apple spice is also popular.
There are special baking pans for Gem Cakes. Older baking pans for Gem Cakes are made of cast iron; newer ones are aluminium. The pan will consist of small rectangular moulds, with convex (rounded) bottoms, all joined together as a tray. As they were classically made of cast iron, the pans are often referred to as “gem irons.” 
You heat the pan first in the oven, then put a bit of butter in to melt, then put your batter in. Gem Cakes cooked in cast iron moulds cook particularly fast, as the pre-heated cast iron holds its heat.
Compare with Australian-style Gem Cakes.
In America, very similar pans were made by the Griswold company, and called “New England Style French Roll Pans.”
Gem Cake Recipe (makes 12)
1 cup of flour
1 teaspoon baking soda
1 teaspoon ground dried ginger
pinch of salt
4 tablespoons brown sugar
2 tablespoons butter
2 tablespoons golden syrup (or corn syrup)
1/2 cup milk
12 mould gem pan
Start oven heating to 425 F / 215 C. Put the gem pan (ungreased) in the oven right away so that it heats, too.
Sift into a large mixing bowl the flour, baking soda, ginger and salt. Stir in the brown sugar. Set aside.
Put the butter and syrup into a small mixing bowl, and zap in microwave just until the butter is melted — about a minute. Take out of microwave, stir to mix with a fork. Add the milk; beat the egg in a separate cup, then add it to the milk and syrup mixture. Add the milk mixture to the flour mixture, and mix well.
Take the gem pan out of the oven. Put a dab of butter into each mould where it will quickly melt (some people spread it around with a pastry brush; others say they never bother.)
Divide the batter up amongst the moulds.
Put the pan back in the oven, bake for about 12 minutes or until a toothpick comes out clean.
Remove pan from oven, let stand for 5 minutes, then turn out onto wire rack.