Preheat the oven to 160ºC /325ºF conventional oven. Prepare a 20x20 cm (8x8 inch) square pan with parchment paper.
In a bowl, sift together the flour, cornstarch, espresso and salt and set it aside.
75 g all-purpose flour, 1 tablespoon cornstarch, ½ teaspoon instant espresso powder, ½ teaspoon salt
Melt the butter and chocolate together. Once melted, pour it into a large bowl and add the cocoa powder and stir it together with a hand whisk or electric mixer.
90 g butter, 150 g semi-sweet dark chocolate chips, 35 g unsweetened Dutch cocoa powder
Then add the brown sugar and granulated sugar, and whisk it together.
130 g granulated sugar, 130 g dark brown sugar
Add the eggs, and mix until incorporated. Once incorporated, continue giving them a good whisk for 1-2 minutes.
3 large eggs
Then add the oil and vanilla extract and whisk until combined.
70 g vegetable oil, 1 ½ teaspoon vanilla extract
Add the dry ingredients to the batter and gently whisk it together until combined. Finally add the mini M&Ms to the brownie batter and gently fold them in using a rubber spatula.
160 g mini M&M’s
Pour the brownie batter into the prepared pan, and smooth it out with the spatula. Top of the brownie batter with the mini M&Ms.
50 g mini M&M’s
Bake for 45-50 minutes or until the brownie has set. You can test if it is done by inserting a butter knife into the middle of the brownie. If it has wet batter on it, give it 5 more minutes but if it has moist crumbs on the knife it's done. Let it cool down completely on a wire rack, before serving.