Line a round 8" cake pan with parchment paper and spray with non-stick spray. Set aside. Preheat oven to 375°.
Melt butter in a large glass mixing bowl in the microwave. Add the chocolate chunks and let sit for 3-4 minutes. Stir together till chips melt, adding additional cooking time if needed. Let cool for 5 minutes.
Whisk the salt, and vanilla into the chocolate mixture. Add the three eggs and whisk till well blended. Stir in the cocoa powder.
Pour batter into prepared pan and spread evenly. Bake at 375° for 25 minutes or till toothpick comes out clean.
Let cake cool in pan for about 10 minutes, then invert onto a cooling rack. Immediately flip right side up onto another cooling rack. Cool completely.
Combine butter and chocolate chips in a glass bowl. Microwave for 1-3 minutes or till melted, stirring every 30 seconds.
Spread glaze over cake. Let sit for several hours till chocolate sets.