In a large mixing bowl, beat the softened butter, brown sugar, and granulated sugar together until light and fluffy.
Mix in the egg and vanilla extract, stirring until fully combined. The dough should look smooth and creamy at this stage.
In a separate bowl, whisk together the flour, baking soda, salt, and cinnamon (if using).
Gently stir in the raisins until they are evenly distributed throughout the dough.
Using a spoon or a cookie scoop, drop rounded tablespoons of dough onto the prepared baking sheet.
Bake for 9–11 minutes, or until the edges turn golden brown while the centers remain slightly soft. Remove from the oven and let the cookies cool on the baking sheet for about 5 minutes before transferring them to a wire rack.
