Melt ½ cup of butter in a saucepan over medium heat until golden and nutty (4-5 mins). Remove from heat, stir in malted milk powder and heavy cream. Stir in the remaining ¼ cup of cold cubed butter until melted.
Whisk the butter mixture, dark brown sugar, and white sugar until smooth. Add the egg, egg yolk, and vanilla; whisk well for 1 minute until thick and glossy.
Fold in the flour, baking soda, and flaky sea salt just until the flour disappears. Do not overmix.
Scoop into large balls, rip them in half, and press the jagged edges facing upward back together. Chill in the fridge for at least 2 hours.
Preheat oven to 350°F (177°C). Bake dough balls 2 inches apart on parchment paper for 9-11 minutes until edges are golden. Top with a pinch of flaky sea salt and let cool.