Marzipan cake

I had this almond sweet from Cyprus that I wanted to use before it expired, and it was too much to just eat by myself, so I thought I'd use it to try out this recipe I found on Nigella's site (if I may be on first-name terms with her).

I'm not certain it was supposed to, but this cake didn't turn fully spongy, especially in the lower-centre part, and it browned faster than I expected. The reason might be that the recipe actually calls for a tube cake pan, but I didn't have that, so I used a plain springform pan; or it could just be the effect of the marzipan.

Ingredients
250 grams unsalted butter (softened)
250 grams marzipan (at room temperature)
150 grams caster sugar
1.25 ml (1/4 teaspoon) almond essence
1.25 ml (1/4 teaspoon) vanilla extract
6 large eggs
10 ml (2 teaspoons) baking powder
150 grams plain flour (sifted with the baking powder)


Directions
Grease/line a 25 cm spring form, and turn on the oven at 440 K (or maybe, 430 K for fan-assisted ovens).

For the marzipan, I used the Cypriot sweet, which was already very soft and a bit runny at room temperature. If not, cut to small pieces, and perhaps give it a couple of blasts in the microwave oven if it's cold. When it's ready, put in the blender (do not use a small blender; mine's a decent size, and it was running at full capacity by the time I put all the ingredients in).

Cut the butter in small pieces and also add to the blender (I actually warmed it up a bit in the microwave to make very soft). Add the sugar, and blend everything together.

Add the vanilla extract and the almond essence (actually, I skipped the almond essence because I didn't have any, but I think the marzipan should be more than enough) and blend again. Add the eggs one by one, and blend in between. Finally, add the flour, blend until everything is well mixed together.

Pour the mixture into the form, and bake in the oven for about 50 minutes or until a toothpick comes out clean (start checking from 40 minutes). Nigella says it should look golden when cooked, but mine was already brown at 20-25 minutes.