Preheat the oven to 350°. Grease a loaf pan and dust it lightly with flour, tapping out any excess. Set aside until ready to use.
Cut 1/4 cup of butter into tablespoon-sized chunks and add it to a small saucepan over medium heat. Add the chopped chocolate. Stir often, until completely melted and set aside to cool.
In a medium-sized mixing bowl whisk together the flour, cocoa powder, baking powder and sea salt. Set aside until ready to use.
In the bowl of a stand mixer fitted with a paddle attachment, beat the remaining 1 cup of butter on medium-high speed for about 1 minute. Alternatively, use a large mixing bowl and a handheld mixer. Slowly add in the sugar and beat on medium-high speed for 2-3 minutes, or until light and fluffly. Add the eggs, one at a time, beating well after each addition. Add the vanilla and sour cream and beat once more.
With the mixer running on low, drizzle in the melted butter and chocolate mixture. Add the dry ingredients in 3 additions, alternating with the half-and-half in two additions, mixing on low after each addition. (So the mixing pattern will go like this: dry ingredients, half-and-half, dry ingredients, half-and-half dry ingredients).
Add the batter to the loaf pan and smooth over the top with an offset spatula or knife. Bake 45-55 minutes or until a toothpick inserted in the center of the cake comes out moist, but not damp with batter.
Do not over bake. Transfer to a wire rack and run a sharp knife along the perimeter of the loaf pan.
Allow to cool 15 minutes in the pan, then gently invert and carefully remove the cake. Allow to finish cooling on the wire rack.