In a large pot heat condensed milk with sugar and butter. Cook for 25 minutes and constantly stir.
When the mixture will be golden brown cook yet for 5 minutes.
Test consistency: in small bowl pour cold water. Drop little bit of hot caramel into the cold water. If you can make small ball from the cold caramel, the fudge is ready.
Let cold the fudge for 5 minutes.
Line a 20x30 cm rectangle cake tin with baking parchment.
Add the vanilla extract to the fudge and whisk it with electric hand whisk until the mixture becomes thick and creamy.
Pour the fudge into the cake tin and let cold for 3 hours, than cut it square.