Bring butter to room temperature either by letting sit on the counter for 10-15 mins before using. Alternatively, you could place the mixing bowl in a bigger bowl of hot water while creaming it with sugar.
In a small to medium mixing bowl, beat sugar and room temperature butter with a whisk until fluffy, creamy and light in color. To this add prune purée, 1/3 cup of reserved prune soaked water, vanilla extract and whisk together.
Prunes can be soaked for a couple of hours instead of boiling.
Do not open the oven door during the first 15 mins as it causes the rising dough to collapse.
Do not forget or delay cooling the cake on a wire rack right after pulling out of the oven. Otherwise, due to the retained heat, cake starts to sweat inside and end up soggy instead.
Make sure there is enough room for the cake to rise or else the dough will spill out during baking.
Do not over-bake or it will become dry and hard.