In a large mixing bowl using a hand mixer, or in the bowl of a stand mixer using a paddle attachment, cream together the butter with the icing sugar on medium speed until smooth and creamy.
Put the chocolate in a microwave safe bowl and heat in 20 second increments until just melted.
Add the chocolate to the butter mixture and sift the cocoa powder into the bowl. Mix to combine.
Add the vanilla, milk, and salt, and mix to combine.
Sift the dry ingredients (flour, baking powder, corn starch, and salt) into the butter mixture and stir until just combined.
Turn the cookie batter out onto a sheet of plastic wrap. Wrap tightly and let cool for an hour or up to overnight.
When you are ready to bake, heat oven to 350 F and line cookie sheets with parchment paper or a silicone baking mat.
On a generously floured surface, roll the cookie dough to 1/4 inch thickness. Use a heart-shaped cookie cutter (we used three different ones in different sizes) to cut out the cookies.
Place the cookies on the prepared baking sheet and bake 10-15 minutes until just set.
Let cool 5 minutes in pan, then turn out to wire rack to fully cool.
Notes
These cookies keep perfectly for several days in an airtight container.