Preheat your oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper, leaving some overhang for easy removal later.
In a large mixing bowl, combine the melted unsalted butter, granulated sugar, and light brown sugar. Whisk until smooth. Then, add the eggs, coffee or water, and vanilla extract. Mix until well combined.
In a separate bowl, sift together the cocoa powder, instant espresso granules (if using), and all-purpose flour. This ensures there are no lumps and helps to evenly distribute the cocoa.
Gradually add the dry ingredients to the wet mixture, stirring gently until just combined. Be careful not to overmix. Fold in the semi-sweet chocolate chips until evenly distributed throughout the batter.
Pour the brownie batter into the prepared baking pan, spreading it out evenly. Bake in the preheated oven for 25 to 30 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs.
Once baked, remove the brownies from the oven and let them cool in the pan for about 10 minutes. Then, lift them out using the parchment overhang and transfer them to a wire rack to cool completely.
While the brownies are cooling, prepare the frosting. In a small saucepan over low heat, combine the half-and-half or cream with the remaining chocolate chips. Stir until melted and smooth. Let it cool slightly before spreading it over the cooled brownies.
While the frosting is still slightly warm, sprinkle the M&M minis over the top, pressing them gently into the frosting. Allow the brownies to set at room temperature for a bit before cutting them into squares.
