There is a plethora of bakeries in Seoul, but I find that most of them offer similar fare -- pastries and buns in a Frenchish vein.
Until I recently got all the fixings to make my own brownies and cookies, I often stopped into bakeries looking for a brownie or chocolate chip cookie. Though not common, I've managed to find a number of brownies and chocolate chip cookies, but they've all turned out to be bland, dry, and definitely overpriced. Let's not skimp on the butter, people!
The only bakery that has come close to making a decent brownie is Queens Park in Apkujung. The selection of pies, cakes, loaves, and pastries all look very lovely, but I have to warn you, they're quite pricey. But, if you're like me, and willing to spend more on baked goods than you do on shoes or clothing, then perhaps Queens Park won't seem like such a bad deal.
In addition to baked goods, Queens Park offers a nice looking brunch and dinner menu. I've only sampled the baked goods, but I imagine that the sit down menu is pretty nice.