Take the unsalted butter in a pan.
Keep the pan on a stovetop and on a low heat melt the butter.
Just melt the butter. No need to heat the butter. Stir so that the butter melts quickly.
Keep the pan down on the kitchen countertop once the butter has melted. Add the sweetened condensed milk. Condensed milk added is about ½ a tin or can of condensed milk measuring 400 grams or 14 ounces.
Begin to stir very well with a wired or balloon whisk.
Whisk very well to make a smooth even mixture.
Now add the mango puree and vanilla extract.
Stir again very well to get a smooth even mixture. Add the sifted flour.
With light hands, gently fold the mixture with a spatula. Avoid folding too much.
The cake batter should have volume and should not fall flat or become doughy like roti or naan dough. Depending on the quality of flour, you might get a thick batter like I have got. This much thickness is fine.
But if the mango cake batter looks very thick, then do add some tablespoons of milk. Also mango puree made at home is usually thick. If you use store bought mango puree, then the batter will be slightly on the thinner side.
With the spatula add the batter to the loaf pan.
Shake the pan to even the batter from the sides. Also with the spatula even the batter from the top.