Step 1

Boil sugar and 100 ml water in a saucepot over medium heat. Continue cooking until the sugar melts and the color is light amber.

Step 2

Reduce the heat to low and continue cooking until it starts to caramelize. The syrup will bubble rapidly and the color turns darker.

Step 3

Immediately take the pot off the heat. After 30 seconds, pour the remaining 250 ml water into the pot. Wait 5 minutes, then stir until all the caramel dissolves.

Step 4

Transfer the syrup to a bowl and cover with a saran wrap. Let it cool in a fridge. (*)

Step 5

Preheat the oven to 180 Celsius (350 Fahrenheit). Grease and flour an 8"x8" square cake pan and set aside.

Step 6

Sift together all-purpose flour, baking soda, and salt in a mixing bowl. Set aside.

Step 7

In a large mixing bowl, cream butter with a mixer with medium speed. Add egg yolks, one at a time, and beat until well combined. Add 1⁄3 of the egg whites (2 egg whites) and continue mixing until well combined, then add the rest of the egg whites (4 egg whites) and continue mixing until well combined.

Step 8

Add vanilla extract into the batter and mix until well combined.

Step 9

Set the mixer to low speed. Add the flour mixture in two batches, beating each addition until well combined. Add sweetened condensed milk, mix until well combined.

Step 10

Pour the cooled caramel syrup into the batter. Fold with a spatula until well combined.

Step 11

Pour the batter into the prepared cake pan. Bake the cake on the lower third of an oven for one hour, or until a cake tester comes out clean.

Step 12

Cool the cake in the pan, then cut into serving slices and serve at room temperature.