This one could just be the recipe's fault. I see now that it has you pour, basically, boiling milk onto eggs and sugar. That's pretty unusual; it could well result in you cooking some of the egg (bad!) before it's all incorporated. In your case, it sounds like it didn't quite go that far: you may have managed to cook the custard just right by accident. It's not helping you, though, and it might result in disasters eventually, especially with smaller ratio of egg to hot milk in another recipe. A more normal process would be:
- heat but don't boil at least some of the the milk and cream with the sugar
- stir some of the heated milk into the eggs to bring them partially up to temperature
- stir the warmed eggs and milk back into the rest of the milk
- continue heating and stirring until thickened (this might take some time, but not 20 minutes)
These steps, for example, are generally what David Lebovitz says to do in the recipes in The Perfect Scoop. He's a well-respected pastry chef who's also become a bit of an authority on ice cream; I've had success with every recipe I've tried from his book. As an example, this brown bread ice cream recipe from his blog follows those steps.
As for the texture of the final ice cream, that might be because of your eggs. Recipes are calibrated for large eggs; jumbo eggs are bigger, probably about 5/4 the size of large eggs, so 7 jumbo yolks might have been about 9 large yolks. The eggs are what give French-style ice cream its smooth, silky texture, so additional egg will accentuate that. I'd try it with 5 or 6 yolks (effectively 6.25 or 7.5).
It sounds like the texture you describe may have also lacked air, which could also be due to a few other common things. This could be due to insufficient egg-beating, but that's not the usual way to get light, fluffy ice cream, so I'd hesitate to blame it. Churning in the ice cream maker (this is not the same as the previous egg-beating) is normally the primary source of air. If you don't churn it long enough in the ice cream maker, it won't be as airy. Assuming you have the type of ice cream maker with the pre-frozen vessel, it's also important that the vessel be cold enough; if the custard doesn't freeze thoroughly in there, it won't be able to hold the air.
The ripple is a separate question. Generally, if you want something like that to stay soft when frozen, the way to do it is to load it up with sugar. The frozen raspberries I've seen aren't in syrup; if yours weren't either, you just didn't have nearly enough sugar. Otherwise, I'd again blame the recipe; you could probably fix it with more sugar. You could also try keeping it in a warmer place (the door) in your freezer. As some related reading, Lebovitz has a great blog post on making ice cream softer - note that sugar content is one of the methods. He also mentions use of stabilizers and anti-crystallization agents in ice cream; the same kinds of things are used in commercial versions of things like this ripple.
So, be wary of online recipes; they're not always the best, and sometimes they're not even well-tested, especially with something like ice cream that most people don't make. And once you get the basics down (e.g. learn the process from someone very trustworthy like Lebovitz), you can take a recipe online, keep the ingredient list, and prepare it the way you know works, ignoring some of the listed steps.
Alcohol Sugar Fat. Your recipe needs to be more of those and less of water; just like an ice cream.
I had luck with agave syrup as a base (maple or cornsyrup?) that I tested by freezing; when I was satisfied by my choice, I melted in chocolate and additional fat (good coconut oil I think).
Certainly won't be fudgy outside of freezer so finding a recipe is tricky...
Have a look also at that ice cream topping chocolate hard-shell. It has a super low melting point with its high fat content. Wouldn't take much more sugar in the form of a syrup to make it fudgy.
Best Answer
When I've done this in the past, I did two things differently.
I used more of a fudge sauce that would thicken and get somewhat firmer when cold. Like a fudge sauce or such. Syrup just mixes too easy.
Mixing a swirl into a deeper container like the churning tub proved somewhat diffcult. As I tried to swirl, it would mix instead. Instead, I spread the ice cream in a large cake pan, poured the fudge over, swirled in a bit with the knife and then set it in the freezer for about 1 hour. Then when everything was pretty firm, I scooped it up into the final container in as large of sections as possible. It worked great.