Making Brownie Ice Cream Sandwiches involves a few essential steps that will guide you to creating these mouthwatering treats. Follow these instructions closely:

1. Preheat Oven: Start by preheating your oven to 350°F (175°C). Prepare a 9×13-inch baking pan by greasing it with butter or lining it with parchment paper.

2. Mix Wet Ingredients: In a large bowl, combine the melted butter, granulated sugar, and brown sugar. Stir until smooth. Add in the eggs, one at a time, mixing well after each addition. Finally, stir in the vanilla extract.

3. Combine Dry Ingredients: In another bowl, whisk together the flour, cocoa powder, baking powder, and salt until well combined.

4. Combine Mixtures: Gradually add the dry ingredients to the wet mixture, stirring until fully combined and no flour streaks remain.

5. Bake the Brownies: Pour the brownie batter into the prepared pan. Use a spatula to spread the batter evenly. Bake in the oven for 25-30 minutes. The brownies should be set but slightly gooey in the center.

6. Cool the Brownies: Once baked, remove the brownies from the oven and let them cool completely in the pan on a wire rack. This step is crucial to prevent melting the ice cream when the sandwiches are assembled.

7. Cut the Brownies: After the brownies have cooled, carefully remove them from the pan and cut them into squares or rectangular shapes, depending on how large you want your sandwich.

8. Assemble Sandwiches: Take one brownie piece and place a generous scoop of ice cream on top. Use another brownie piece to top it off, pressing slightly to seal the sandwich.

9. Freeze the Sandwiches: Place the assembled sandwiches on a baking sheet lined with parchment paper and freeze for at least an hour. This ensures they hold their shape.

10. Serve: Once frozen, remove the sandwiches from the freezer. You can drizzle them with chocolate syrup and add whipped cream if desired before serving.