Skinny Peanut Butter Swirl Brownies

creamy peanut butter1

low fat or fat free plain or vanilla Greek yogurt

milk (I like to use unsweetened almond milk)

large egg (or 2 egg whites)

salt

baking powder

granulated sugar or coconut sugar (see note above in post about substitutions)

unsweetened natural or dutch process cocoa powder

old-fashioned rolled oats2