Step 1

Preheat oven to 350°F. Line an 8x8-inch square metal baking pan with parchment paper leaving a 2-inch overhang at each side and secure the sides down on the edge of the pan with ¾-inch binder clips. See notes on the last step for baking in a 9-inch pan.

Step 2

Make the cookie dough. Melt the butter in the microwave or on the stovetop until it is JUST melted so it is still pale yellow and creamy-looking. Basically, gently melt it until there is still some solid butter left and then stir off the heat until the residual heat gently melts the remaining solid butter. Combine melted butter with brown sugar and granulated sugar in a large bowl and mix to combine. Mix in egg yolk and vanilla. Sprinkle in flour, baking soda and salt and fold it in. Fold in chocolate chips.

Step 3

Roll cookie dough into about 18 small balls, transfer them to a lined plate or tray, cover and refrigerate while you make the brownie batter.

Step 4

Make the brownie batter. Combine chocolate and butter in a saucepan over very low heat and stir constantly until melted and smooth, or melt in the microwave at 50% power in short bursts with frequent stirring.

Step 5

Combine eggs, sugar, vanilla and salt in a large bowl and whisk until pale and thick. Beat in warm melted chocolate mixture. Sprinkle flour over the mixture and fold it in until evenly combined.

Step 6

Scrape about half of the brownie batter into prepared pan and spread it out evenly using a small offset spatula. Flatten half (of the chilled dough balls into discs and lay them over the batter in three rows of three. Spread remaining brownie batter on top to cover the cookie dough, and then place remaining flattened dough balls over top. Bake for 20 minutes, then reduce temperature to 325°F and bake for another 15-20 minutes until a skewer comes out with a few moist stick crumbs attached.

Step 7

NOTE: you can also bake in a 9x9-inch pan and in this case, place half of the cookie dough balls directly onto the bottom of the lined pan, spread over all of the brownie batter, then place the remaining dough balls on top. Bake for about 30 minutes at 350°F.