In a medium bowl, whisk together the 2¼ cups all-purpose flour, 2 tablespoons malted milk powder, 1 tablespoon corn starch, and ½ teaspoon baking soda.
In the bowl of a stand mixer fitted with a paddle attachment, cream together 1 cup light brown sugar, ¾ cup salted butter (melted), ½ cup granulated sugar, 1 tablespoon vanilla extract, and 1 teaspoon salt until smooth. 1 min on 2 and 1 min on 4 speed.
Add in 1 large egg and 1 additional large egg yolk and beat until light and creamy. About 5 minutes.
Add the dry ingredients to the wet ingredients a little at a time, mixing between each addition.
Chop 9 ounces milk chocolate bar and fold in most of the chopped chocolate into the dough. Reserve the rest for the top after baking.
Roughly chop 1 cup salted dry roasted macadamia nuts and add to the dough, mixing just until incorporated.
Measure the dough into 1/4 cup-sized balls, or about 3 ounces each. I use a #16 cookie scoop. If desired, press additional macadamia nuts into the top of the dough.
Chill uncovered in the fridge for at least 2 hours, preferably 24.
Preheat oven to 325°F and place balls on a parchment-lined cookie sheet at least 3 inches apart. Bake for 15 to 17 minutes until the edges are golden brown but the centers still look soft but not glossy.
As soon as the cookies come out of the oven, use a large (4-inch-ish) cookie cutter or bowl to swirl them around one at a time to shape them. Then, immediately press the remaining chocolate chunks into the hot cookies and let the cookies cool on the pan for 10 minutes before transferring to a wire rack to cool.
Once the added chocolate warms up, sprinkle with flaky sea salt if desired.