In a medium bowl, whisk together the flour, cocoa powder, chocolate malted milk powder, espresso powder, and salt. Set aside.

In a large heatproof bowl, add the butter and chopped dark chocolate. Heat in the microwave in 20 second intervals, stirring in between. When it's 80% melted, stir until smooth and combined. The residual heat should melt the rest.

Whisk the granulated and brown sugar into the chocolate mixture until fully combined. Let the mixture cool for about 2-3 minutes or until it's slightly warm. If it's too hot, the heat could cook the eggs.

Once the chocolate mixture has cooled slightly, whisk in the eggs and egg yolk one at a time. Mix after each addition. Whisk in the vanilla extract.

Fold in the flour mixture with a rubber spatula until just combined. Be careful not to overmix. The batter will be slightly thick.

Transfer the batter to the prepared baking pan, smoothing out the top.

In a small bowl, mix together the peanut butter and powdered sugar until combined. Heat in the microwave in two 5 seconds intervals, stirring in between (for a total of 10 seconds). It doesn't need to be runny. Just warm enough to fall off the spoon in thick ribbons.*

Place dollops of peanut butter on top of the brownie batter. Then take the tip of a thin knife or a chopstick and swirl the top until you're satisfied with how it looks. Don't neglect the edges!

Place the pan on a cooling rack and cool completely in the pan before slicing.

Top with flaky sea salt and enjoy!