Oh my gosh guys, I just found this and I had to post it.
I've tried the Chocolate Muffin in a minute version of this so I know it will work and I'm so excited to try it!

http://www.food.com/recipe/very-low...rger-bun-229625

2 tablespoons melted butter
pinch of salt
1 big egg
4 tablespoons flax meal
1/2 tsp baking powder

Mix well in a bowl the size you want your bun to be. Nuke 1 minute or until centre has set. Cool, halve and fill.
There are only around 2g of carbs in this bun between the flax & baking powder, so it's a good fall-back if you're sick of eating burgers with nothing around them!