Preheat oven to 350 degrees F (180 C). Set aside a 9×9-inch baking pan. Line the interior with parchment paper or foil.

In the microwave at 50% power, or in a medium saucepan set over low heat, melt together the chocolate and butter, stirring frequently.

When fully melted and smooth, remove from heat. Add the sugar, espresso powder, and eggs, whisking vigorously to fully incorporate.

Add vanilla and sea salt and whisk again.

Sprinkle flour over the batter. Stir with a spoon or spatula just until combined, then gently mix in chocolate chips and M&Ms.

Scrape batter into the prepared pan, pressing batter into the corners if necessary, and dot the top with a few more M&Ms.

Bake for 25-30 minutes, until a toothpick inserted into the center comes out with just a few crumbs.

Remove from the oven and let cool fully to room temperature.

Lift brownies from the pan using the parchment or foil liner, and place on a cutting board. Cut into shapes with cookie cutters, and serve.