Preheat oven to 180°C (350°F or Gas 4).

Line a 8x8-inch baking pan with aluminium foil and spray with baking spray.

In a heat-proof bowl over simmering water, melt butter, sugar, cocoa powder, and salt.

Stir occasionally until melted and slightly gritty.

Remove from heat and cool for 5 minutes.

Stir in vanilla extract.

Mix in eggs, one at a time.

Fold in flour until just combined.

Pour batter into prepared pan and spread evenly.

Bake for 20-25 minutes, until a toothpick inserted into center comes out almost clean.

Cool completely in the pan on a wire rack.

For the cookie dough, beat softened butter and sugar until light and fluffy.

Beat in milk and vanilla extract.

Add flour and salt, and mix until combined.

Fold in chocolate chips.

Gently spread cookie dough over cooled brownies.

Refrigerate while preparing the chocolate layer.

For the chocolate layer, melt butter and chocolate together in the microwave, stirring every 30 seconds.

Pour chocolate over the cookie dough and spread evenly.

Refrigerate for 2-4 hours (or overnight) until firm.

Cut into squares and serve.

Keep refrigerated in an airtight container for up to 3 days.