As a Pavlova and coffee addict I had to try the Cappucino Pavlova recipe. Whisking the egg whites to firm peaks went fine, then adding the sugar coffee mixture made the whites even firmer. In the end adding the cornflour and white wine vinegar. Maybe the folding did not go well but all seemed fine. Spreading the mixture on the plate, tough but doable. After an hour in the heated oven, following the recipe and leaving it to cool. The next morning it looked great, a nice disc with an height of 3 to 4 cm. But once out of the oven, the dome collapsed, leaving a gooey layer of about 9 mm. The outer crust tasted great as a hard meringue. But the inside was not as I was expecting. I was expecting somewhat like marshmallow structure. Where could I have gone wrong. I will certainly go for it again, but hope to get it right then.
The espresso powder added to Nigella's Cappucino Pavlova (from Nigellissima and on the Nigella website) will slightly change the structure of the pavlova, meaning it is not quite as marshmallowy as a regular pavlova, but it should not leave a thin, gooey centre to the meringue.
From the description we suspect that the egg whites may have been overbeaten. The recipe suggests only beating the egg whites to soft peak stage (when you lift the whisk the egg white on it will form a small peak which flops over at the top) and not to firm peaks. When the sugar has been added the meringue should be soft and spreadable and not firm.
It is also possible that the meringue went into an oven that was too hot, causing a very hard shell but a wet and undercooked centre.