Preheat the oven to 300°F. Line a rimmed baking sheet with parchment paper.

Spread the nonfat dry milk powder into a thin, even layer.

Bake for 10 minutes, then stir thoroughly and break apart any clumps.

Continue baking for another 10–20 minutes, stirring every 5 minutes, until the milk powder is deeply golden beige and smells noticeably toasted and nutty.

Remove from the oven and cool. If the toasted milk powder is clumpy, pulse briefly in a food processor or blender or press it through a fine mesh sieve. Set aside.

Increase the oven temperature to 350°F for the blondies.

Heat the heavy cream until steaming but not boiling.

Whisk 2 tablespoons toasted milk powder into the warm cream until as smooth as possible. Keep warm.

Add the granulated sugar to a medium light colored saucepan over medium heat. Allow the sugar to begin melting around the edges, then gently stir or swirl as needed until completely melted and deep amber.

Remove the pan from the heat and carefully whisk in the butter.

Slowly pour in the warm toasted milk cream while whisking continuously. The mixture will bubble vigorously, so add it gradually.

If necessary, return the pan to low heat for about 1 minute and whisk until completely smooth.

Remove from heat and stir in the kosher salt and vanilla.

Set aside to cool slightly while you make the blondie batter. The caramel should still be pourable but should not be extremely hot when added to the batter.

Line an 8×8 inch metal baking pan with parchment paper, leaving enough overhang to lift the blondies from the pan after cooling.

Add the butter to a light colored saucepan over medium heat. Melt and continue cooking as it foams. Stir or swirl frequently until the milk solids become deeply golden brown and the butter smells nutty.

Immediately remove from the heat and whisk ⅓ cup toasted milk powder into the hot brown butter.

Allow the mixture to cool for 5–10 minutes. It can still be warm but should not be piping hot.

Add the dark brown sugar and granulated sugar and whisk until thoroughly combined.

Add the eggs and vanilla. Whisk vigorously for 45–60 seconds, until the mixture is smooth, glossy, and cohesive.

Add the flour and kosher salt. Switch to a rubber spatula and fold until just combined and no dry streaks of flour remain. Do not overmix.

Spread the thick batter evenly into the prepared pan.

Dollop approximately ⅓ cup salted toasted milk caramel over the surface.

Run a butter knife or skewer through the caramel using a few broad figure eight motions. Do not completely mix the caramel into the batter. You want distinct ribbons and pockets.

Bake at 350°F for 27–34 minutes. Begin checking at 27 minutes. The edges should be set, the center should no longer appear raw, and a toothpick inserted away from a caramel pocket should come out with moist crumbs.

Cool in the pan for at least 45 minutes before lifting out and cutting.

Cut into 16 squares. Drizzle with reserved salted toasted milk caramel and finish with flaky sea salt.