Preheat oven to 400°F. Line a baking sheet with parchment paper.
Fill a pot about halfway full of water. Bring it to a simmer over low heat. Find a heat-safe bowl (glass or stainless steel) that can be set into the pot of simmering water. Make sure the bottom of the bowl is not touching the water. The steam is what will melt the chocolate.
Set aside ¼ cup of chocolate chips. Add the rest of the semi-sweet chocolate chips and ¼ cup (½ stick) of butter to the heat-safe bowl. With the heat on low, use a rubber spatula to stir the chocolate until it is melted and smooth. This takes about 5 minutes. Set aside.
In a large mixing bowl, whisk together flour, baking powder, baking soda, salt, and rolled oats. Set aside.
To the bowl of a stand mixer with the paddle attachment, add the remaining butter and brown sugar. Beat on low speed until smooth.
Add melted chocolate, eggs, and vanilla. Mix until all ingredients are fully incorporated, stopping to scrape down the sides of the bowl as needed.
With the mixer still on low speed, slowly add the dry ingredients until fully incorporated.
Using a 2-tablespoon scoop, drop the dough onto the lined baking sheet. Roll each into a ball. Then, gently press to slightly flatten the cookies. Top with the reserved chocolate chips.
Bake for 7-8 minutes. They may look slightly doughy, and that is okay.
Let the cookies rest on the baking sheet for a couple of minutes before transferring them to a wire rack to cool completely. Enjoy!