From the recipe and your "fudge" description, I think the problem was that you didn't get enough air in the batter. Trying a different recipe is a good idea.
The way to get more air into a flourless cake is through the eggs. Look for recipes which require you to beat the egg whites separately to soft peaks. Mix everything very carefully, in the correct order. If your try still doesn't get you enough air, try more tricks. Whip your yolks with some liquid (maybe 10 ml per yolk) over a water bath (keep it between 60° and 70°C!), practically creating zabaglione. But really watch the temp with a probe thermometer - you don't get a good emulsion below 60°C, and you don't want your proteins to coagulate while whipping, they must do it latter in the oven (they start a bit above 70°C).
The other problem is that you might get too smooth a texture. First, select a recipe with less fat. Fat makes the batter softer, but also very smooth. Your hunch is correct: Choose a recipe which calls for both cocoa powder and chocolate. The powder has much less fat, resulting in a higher percentage of starch, and that creates a more cakelike texture. Another good way to change the texture is to use recipe which includes a nut flour. A raw almond flour will give you very little change in flavor, but if you don't mind deviating from the original, roasted hazelnut pairs very well with both chocolate and raspberry.
And as we are speaking of fat: you must use a good chocolate! Standard chocolate bars (Hershey, Milka) are no good. To cut costs, they use vegetable fat instead of cocoa butter, which changes the texture a lot. Then there are the manufacturers who use emusifiers and include even less cocoa butter. If you can get hold of real confectioner's chocolate (Valrhona, Callebaut), use it. If not, buy a chocolate bar which contains only cocoa mass, cocoa butter and sugar and nothing else - no vegetable fat, no emusifiers, etc. Also no milk solids, you want to make this kind of recipe with bittersweet to bitter chocolate. The one big brand I use for baking is Lindt Excellence (not Lindt Lindor!), but there are also some supermarket-branded chocolates with the right contents.
I don't have a tried recipe to offer you, but I am sure that the usual suspects have published something good. I think Smitten Kitchen had a nice big post on flourless pastries ones (but don't remember if they had a choco cake there), and Lebovitz and/or Desaulniers probably have recipes for a flourless choco cake too.
Speaking from making fudge, I have gotten the most consistent results using superfine / castor sugar.
- I haven't noticed starch related issues from powdered sugar, but your mileage may vary; the problem I have had with it is due to clumping in spite of the starch (particularly in a frosting recently).
- I have used granulated sugar to make fudge and it is grainy. I melted most of it, but got sick of stirring it as it got crazy thick. I suppose it's possible to get it to the point of a completely dissolved syrup, but that would take longer than it is worth since you are likely going to run the risk of scorching.
Relative to the taste and structure, you won't get anything special from the caster sugar. However, you will get a more predictable product, and less hassle waiting for it to dissolve. You will also avoid texture problems and reduce the risks brought on by extended time in the pot (i.e. too much heat, likelier to accumulate moisture randomly).
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At Marriott hotel, where I worked for a while, we used similar flakes. Our technique was to add a good splash of boiling water first, incorporate the chocolate flakes by mixing it into a smooth paste (more or less), and only then add hot milk. I guess the idea is that it is much more of a challenge to incorporate a small amount of solid into a large amount of liquid, however if you turn that solid into a paste first, its almost like your mixing two liquids. You'll find that the same technique is used for making a bachamel sauce. For convenience sake, try using a small amount of hot milk, instead of water. Also, having your liquid boiling might help, which is hard to achieve in a microwave. e.g. when making a classic chocolate sauce which contains butter and cream, the chocolate is only fully dissolved when the liquid is boiled.