Preheat the oven to 350°. Butter the edges of a heavy sheet pan. Cut a piece of parchment paper large enough to come just above the edges of the pan. This is for removing the squares neatly for cutting. You can just butter the whole pan, but the squares won't look as neat.
In a large bowl, beat together the butter and sugar until fluffy.
Add vanilla and eggs and beat again until creamy.
Add the flour, coconut, salt, and baking powder and mix until combined. It will be thick.
Spread evenly in the pan. You may have to wet your hands and pat it out – if it begins to stick to your hands, just wet them again, until the dough is spread out to the edges.
Bake for 20 to 25 minutes, until a toothpick inserted near the center comes out with no more than a few moist crumbs.
Remove from the oven and immediately distribute 2 cups of bittersweet chocolate chips over the top. Let sit for 5 minutes, which will melt the chips, and then spread evenly over the bars, with an offset spatula. Cool in the pan for 30 minutes or so [longer is fine, but the edges might stick a tad more in that case].
After 30 minutes, loosen the edges and use the parchment paper to lift the squares out in one piece. Place on a cutting board, and before they cool completly, use a large sharp knife to cut straight down through the bars. Dragging the knife will make the chocolate look messy.
Cut in whatever shape you prefer. They're on the rich side so don't make them huge. Allow to cool completely before storing in a sealed container.