Preheat oven to 350 degrees Fahrenheit. Line an 8x8-inch baking pan with parchment paper.
In a medium saucepan melt 3/4 cup salted butter. Stir in 1 1/2 cups granulated sugar until combined.
Remove the pan from the heat and stir in 2/3 cup unsweetened cocoa powderuntil completely blended.
Add in 1 teaspoon vanilla extract, then working one at a time, stir in 3 large eggs, stirring well after each addition.
Slowly mix in 1 cup all-purpose flour until combined then fold in 1/2 cup of your M&M's.
Pour into prepared baking dish and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean. Transfer to a wire rack to cool completely.
Make your ganache by pouring 1 1/2 cup milk chocolate chips into a small mixing bowl. Pour 1/2 cup heavy cream into a small saucepan and heat over medium-high heat until it just starts to simmer. Immediately pour the hot cream over the chocolate chips. Let sit 5 minutes, then stir until smooth.
Pour your cooled brownies and sprinkle with the remaining 1/4 cup M&Ms.
Chill in the fridge for 30 minutes or until the ganache has set.