Idil T.
Yelp
I've had kanel & kardemummabullar from Stockholm's well-frequented spots, like Fabrique, and lesser known, like Saturnus, and Petrus takes the cake (or dare I say bread) everytime.
If you are arriving from the south (i.e. arriving from Södra station), try to not get lured by Johan & Nyström's; go their for your caffeine fix at some later time, surely, but at the moment, direct all attention to Petrus. But try to not walk past it; because the café is what Swedes would refer to as "mysigt" (cozy), you could unknowingly walk past it and stride into Mariatorget.
Upon entering, you will be tasked with placing an order from the all equally eye-catching buns. Reassuringly, know that you can do no wrong, but if it's your first time (and one of your first times having a fika), the Beginner's Guide to Fika (if there were one) would recommend a kanelbulle, or a cinnamon bun. It's sticky, it's doughy, it's sweet, and the cinnamon is all there, with a kick of cardamom. It's best complimented by a cup of tea, I think, because coffee might steal the spotlight from the flavor. For seasoned fika experts, a kardemummabulle will give you an even stronger kick of spice. The kanel and kardemumma "fikabrod" are like the buns, just made in a bigger quantity.
The semlor (available from Jan-April) are also great, but know that they aren't the most conventional. They're a bit sweeter than normal, my hunch being that there's something in addition to the almond paste that's usually in the buns. Perhaps it's a little too sweet, but nothing that irritated me, or dare I say stopped me.
Seating can be a bit tricky to find, but don't let that discourage you from coming here. Staff are always super friendly and kind.
I wish I lived next door.