In a medium bowl, whisk together 2¼ cups all-purpose flour, 2 tablespoons malted milk powder, 1 tablespoon cornstarch, and ½ teaspoon baking soda.
Melt ¾ cup salted butter in a small butter pot.
Meanwhile, add 1 cup light brown sugar, ½ cup granulated sugar, 1 tablespoon vanilla bean paste, and 1 teaspoon fine sea salt to the bowl of a stand mixer fitted with a paddle attachment.
Pour the melted butter over the sugar and mix for 1 minute at speed 2, then 1 minute at speed 4.
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 in three parts, mixing between each addition.
Fold in most of the 1 cup milk chocolate chips and 1 cup mini semi-sweet chocolate chips, reserving some for topping.
Measure the dough into 7 balls, or about 6.2 ounces each and arrange the balls in a 9-inch pie dish, with one ball in the center and the rest around it. If desired, press additional chocolate chips into the top of the dough.
Chill uncovered in the fridge for 2 hours.
Remove the cookies from the oven and let them sit at room temperature while the oven preheats to 350°F.
Bake for 25 to 30 minutes until the edges and tops are golden brown. Remove from the oven, sprinkle with flaky sea salt, and let them cool for 10 to 15 minutes before serving.
Use an ice cream or cookie scoop to scoop out the cookie portions and serve with a scoop of ice cream and any toppings you desire. The cookies are medium-rare; the center will look underdone and gooey, and the tops and sides should have a bite of chew to them.