Preheat oven to 175°C (350F) and line an 8x4" loaf tin with parchment paper.
Place butter and dark chocolate chunks in a microwave-safe bowl. Heat on high at 20 second increments, stirring well after each heating round until butter and chocolate are melted and smooth. Set aside and allow to cool for 10-15 minutes before using.
Sift and mix flour, cocoa powder, salt, baking powder and baking soda in a medium sized bowl and set aside.
Place eggs in a large bowl of a stand mixer fitted with the paddle attachment. Beat on medium for 1 minute. Add vanilla extract, sugar, and cooled butter-chocolate mixture and mix for another 1-2 minutes until well combined. Mixture should be slightly thick and fudgy.
Add dry ingredients, alternating with milk, in two additions and mix until just combined. Do not overmix.
Pour batter into loaf tin and bake for 45-55 minutes or until a skewer inserted into the center comes out clean. In the meantime, prepare mascarpone frosting.
Allow cake to cool for 5-10 minutes in loaf tin, then transfer to a wire rack to cool completely. Do not frost cake until its completely cooled down.