Prep the oven and baking pan. Position a rack in the center of the oven and preheat the oven to 350°F. Lightly spray an 8-inch square pan with cooking spray and line with parchment paper, leaving a 2-inch overhang on each of the pan's sides. Spray the parchment paper, too.
Mix the dry ingredients. In a medium bowl, whisk together the flour, milk powder, and salt.
Bloom the cocoa powder. In a large heatproof bowl, whisk together the cocoa powder and hot water until smooth.
Melt the dark chocolate, butter, and oil. Add the dark chocolate, butter, and oil to the bowl with the cocoa powder and water. Place the bowl over a medium, heavy-bottomed saucepan filled with a few inches of simmering water (be sure the bottom of the bowl does not touch the water).Cook over medium heat, using a heatproof rubber spatula to stir the mixture and scrape the sides of the bowl occasionally, until the chocolate and butter have melted and combined, about 5 minutes. Remove the bowl from heat, seat on a wire rack, and cool to lukewarm. Add the sugars, eggs, and vanilla. Once lukewarm, whisk the sugars into the chocolate mixture. Add the egg, egg yolks, and vanilla and whisk until combined.
Add the dry ingredients and milk chocolate. Sprinkle the dry ingredients over the batter all at once and use a rubber spatula to mix until combined. Once combined, continue mixing for 1 minute more. Add the milk chocolate and mix until incorporated throughout the batter. Assemble the brownies. Pour the batter into the prepared pan and use an offset spatula to smooth the top. Sprinkle the chopped milk chocolate for the topping on top of the batter. Bake the brownies. Bake for 25 to 30 minutes, or until a skewer inserted into the center of the brownies comes out with a few crumbs attached.
Cool the brownies. Cool completely on a wire rack, about 1 hour, then refrigerate until the bottom of the pan feels cold, about 1 hour more. Serve and store. Run a butter knife or offset spatula along the edges of the pan and use the overhanging foil as handles to lift the brownies out of the pan and onto a cutting board.Slice into 2-inch squares, garnish with flaky salt, and serve.The brownies can be stored in an airtight container or zip-top bag at room temperature for up to 3 days.