Preheat your oven to 350°F. Line baking sheets with parchment paper or lightly grease them. In a mixing bowl, whisk together the flour, baking powder, baking soda, cinnamon, and salt. Set aside.
In a separate large bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy using a hand mixer or stand mixer. Add the egg and vanilla extract to the creamed butter and sugar mixture. Beat until well combined.
Gradually add the flour mixture into the butter mixture, mixing until just combined. Fold in the steel-cut oats until evenly distributed throughout the dough. If using, add in the raisins or chocolate chips and chopped nuts.
Using a spoon or cookie scoop, drop rounded tablespoons of dough onto the prepared baking sheets, leaving space between each cookie for spreading. Place the baking sheets in the preheated oven and bake for 12 minutes, or until the edges of the cookies are golden brown.
Once baked, remove the cookies from the oven and let them cool on the baking sheets for a few minutes before transferring them to a wire rack.