Spray the bottom of a 9-inch springform pan with Pam and cut out a parchment paper circle to put in the bottom. Now spray the sides and the parchment covered bottom again.
Melt butter and chocolate chips for 1 minute in the microwave and stir. If the chocolate isn't completely melted, microwave again in 20 seconds intervals and stir after each increment. Cover bowl to keep it warm.
In another bowl, beat ½ cup sugar and egg yolks together. Add in vanilla.
Add just a little of the egg mixture to the chocolate and stir until combined to temper the eggs. Add all of the chocolate mixture to the egg mixture and stir with a spatula.
In a clean bowl, beat the egg whites on high speed in an electric mixer until you have soft peaks. Add the remaining ¼ cup of sugar and beat until you have stiff peaks.
Add a little of the egg white mixture to the chocolate and egg yolk mixture and stir until combined. Add all of the chocolate mixture to the egg whites and fold the egg whites in very gently with a rubber spatula being careful not to deflate the batter.
Pour the batter into the prepared pan.
Bake for about 35 minutes or until a toothpick comes out clean.
Let it cool in the pan for about 15 minutes. Unlatch the latch on the side of the springform pan. Invert the cake onto a plate and remove the bottom piece of the pan and then remove the parchment paper. Now invert it onto a serving platter.
Voilà!
Recipe by Birch Tree Baking at https://www.birchtreebaking.com/flourless-chocolate-cake/