In a large, non-reactive saucepan over medium-high heat, combine water, key lime juice, kaffir leaves, half the butter and half the amount of sugar. Bring to a boil and let simmer until sugar completely dissolves. Strain out the kaffir leaves and return liquid to pot. In a large bowl, combine eggs, yolks and remaining sugar. Whisk to combine. Temper egg mixture and combine two mixtures in pot. Over medium heat, stir constantly until thickened to curd consistency, scraping the bottom and sides of pot so eggs do not overcook. Have a mesh strainer set up over a bowl within a bowl icebath and force curd through strainer into bowl. Add remaining butter to curd, whisking until butter is fully incorporated. Store Kaffir-Key Lime Curd in fridge with vented plastic film touching curd so a skin does not form and chill well.