Preheat the oven to 350 degrees. Rub a 9x13 glass dish with butter.
Beat the eggs and sugar in a mixer until slightly thickened and light in color, about 7 minutes. The mixture should almost double in size. The eggs are the leavening agent in this recipe, so add time as needed. The mixture should ribbon when the beaters are removed.
Add the butter and vanilla and mix for two more minutes. Mix in the flour until just combined.
Lastly, add the cranberries and mix until combined. Some berries will pop.
Spread into the pan and bake for 40-50 minutes, or until very lightly browned and a toothpick comes out clean.