In a medium bowl, whisk together the melted butter and sugar. Add the eggs and vanilla extract, then whisk until smooth.

Add the cocoa powder, salt, and flour to the bowl. Stir until just combined. Do not overmix.

Spread the brownie batter evenly into the bottom of the prepared pan.

Mix in the egg and vanilla extract.

Gradually add the dry ingredients to the cookie mixture. Once the dough forms, fold in the chocolate chips and half of the Valentine M&Ms.

Using your hands, break off small clumps of dough and drop them over the top of the brownie batter. Press the remaining M&Ms and heart sprinkles into the top of the cookie dough clumps.

Let the brookies cool completely in the pan before slicing. This ensures clean cuts.