Line a 9-inch square baking tin with parchment paper, or spray with non-stick cooking spray.
Place the butter and dark chocolate in a heatproof bowl.
Fill a small saucepan approximately ¼ full with water. Bring to a simmer over low heat.
Sit the bowl with the chocolate over the saucepan and stir until the butter and chocolate have melted. Remove from the heat.
Pour the melted chocolate mixture, brown sugar, eggs and vanilla extract into a mixing bowl, and mix until well-combined.
Stir the flour and cocoa powder into the mixture just until combined.
Fold in 1 cup of the Rolo chocolates.
Pour the batter into the prepared baking tin.
Top with the remaining ½ cup of Rolo chocolates.
Bake for 30 minutes.
Remove from the oven and cool completely in the tin.
Cut into 16 squares for mini-brownies, or 9 squares for larger ones.
Notes
You can substitute white sugar for brown sugar if desired. However, the brownies will be a little less chewy.
The chocolate and butter combination can also be melted in the microwave. To do this, heat for 30-second intervals on low power, stirring in between each one until the two have melted.
Store in an airtight container at room temperature for up to 4 days. Or freeze for up to 3 months.