Preheat your oven to 350°F (175°C).

In a mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.

Add the egg and vanilla extract, and mix until well combined.

In a separate bowl, whisk together the flour, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fold in the chocolate chips (or berries) and oats, if using.

Using a tablespoon, drop cookie dough onto a baking sheet lined with parchment paper.

Flatten the dough slightly with the back of a spoon.

Bake in the preheated oven for 10-12 minutes, or until the edges are golden.

Remove from the oven and let cool on the baking sheet for a few minutes before transferring to a wire rack.