Heat milk, instant coffee, and chocolate until fully melted and stir to combine. Set aside to cool.

Using a hand mixer or simply a hand-whisk, cream the butter for 1 minute until fully-softened. Add sugar and beat until fluffy and lightened in color. Frequently scrape the sides and bottom of the bowl with a silicone spatula for even mix.

Lightly beat the eggs in a separate bowl. Then add them into the butter mixture in 3 additions. Mix just until combined after every addition. If we add all eggs at once, both of these high-fat ingredients might not combine but end up separating.

Add all of the dry ingredients. Make sure to sieve the flour, baking powder, and cocoa powder beforehand as they tend to clump. Mix until 90% combined.

Add the melted chocolate. Mix just until combined. Do not overmix as it will develop the gluten too much and result in a tough cake.

Transfer into a loaf pan that has been fully coated with butter. This will help unmold the cake easily.

Spread up the batter to both ends of the pan. Pipe a line of softened butter along the center lengthwise. These steps are optional, but they will help create an evenly-centered crack.

Bake in a preheated oven at 340°F (170°C) for 40 minutes or until a toothpick inserted in the center comes out clean.

Run a spatula between the cake and the pan. Unmold the cake immediately after baking to prevent shrinking. Let cool on the cooling rack. With a sifter, garnish the top with icing sugar before slicing (optional).