Preheat oven to 350°F.In the bowl of an electric mixer, cream the butter, peanut butter and brown sugar until smooth and creamy. Beat in the eggs and vanilla. Add the mashed banana and mix until just combined.Mix in the flour, baking soda and salt. Add the oats, flax and brewer’s yeast until well combined. Stir in the chocolate chips.Drop heaping balls of dough onto greased baking sheets. Bake for 12 minutes, rotating sheets halfway through, until cookies are set and starting to brown. Let cool for 5 minutes on cookie sheets, then transfer to wire racks to cool completely.