1. Preheat the oven to gas mark 3/ 325 degrees F. Grease a baking pan with butter and set aside.
2. In a bowl, mix together digestive biscuits, caster sugar and butter. Place that into the prepared pan and bake in oven for 10 minutes.
3. Take out the base and let it cool. Meanwhile in a bowl, whisk egg yolks and drizzle in the condensed milk.
4. Add in the lime juice and lime zest and whisk again. Place in refrigerator.
5. Pour the filling over the base and bake again for 10 – 15 minutes.
6. Take out the pie from the oven and let it cool and place in fridge for 2 hours to overnight.
7. Serve key lime pie with cream and enjoy!