Step 1: Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking pan or line it with parchment paper for easy removal.
Step 2: In a medium saucepan, melt the butter over low heat. Remove from heat and stir in the sugar, eggs, and vanilla until well combined.
Step 3: Add the cocoa powder, flour, salt, and baking powder. Mix until just combined. Pour the brownie batter into the prepared baking pan and spread evenly.
Step 4: In a large bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy. Beat in the egg and vanilla extract until well combined.
Step 5: In another bowl, whisk together the flour, baking soda, and salt. Gradually add this mixture to the wet ingredients, mixing until just combined. Fold in the chocolate chips.
Step 6: Drop spoonfuls of the cookie dough over the brownie layer in the pan, spreading it as much as possible without fully mixing the layers.
Step 7: Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out mostly clean.
Step 8: Allow to cool in the pan before cutting into squares and serving.
Notes
Store in an airtight container for 3-5 days or freeze for up to three months.