In a large bowl, whisk together egg, yogurt and sugar until smooth. Add in melted butter and peanut butter slowly, whisking constantly until creamy. Whisk in milk. Add dry ingredients and chocolate chips, mixing until just combined with a large spoon. Do not overmix! Using an ice cream scoop, 1/4 cup measure or large spoon, fill muffin liners 3/4 of the way full. Bake for 15-18 minutes, just until tops are set and slightly golden. Remove and let cool, then serve!