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Begin by preheating your oven to 350°F (175°C); prepare two baking sheets by lining them with parchment paper or a silicone mat.
In a medium-sized bowl, combine the flour, baking powder, baking soda, salt, and cinnamon using a whisk until well blended.
In a larger bowl, utilize a mixer on medium speed to beat together the butter and sugars until the mixture is light and fluffy.
Incorporate the egg into the mixture, then add the mashed banana and vanilla extract, mixing until smooth.
By hand, fold in the flour mixture along with the oats, stirring gently until just combined and no flour streaks are visible; then add the chopped walnuts.
Using a heaping tablespoon, scoop the dough and place it onto the prepared baking sheets.
Bake in the preheated oven for approximately 10 to 12 minutes, or until the edges of the cookies turn a light golden brown.
Allow the cookies to cool on the baking sheet for 3 to 4 minutes before transferring them to a wire rack to cool completely.
Store your cookies in an airtight container to resist the temptation of consuming them all at once!