Preheat the oven to 350°F (175°C). Grease a 9-inch square baking pan thoroughly with butter or non-stick cooking spray. For easier removal, line the pan with parchment paper, allowing an overhang on two sides.

In a microwave-safe bowl, combine the unsalted butter and unsweetened cocoa powder. Microwave in 30-second intervals, stirring after each interval, until the mixture is smooth and glossy. Once melted, stir in the granulated sugar, espresso powder (if using), and vanilla extract until fully incorporated.

Add the eggs to the cocoa mixture, one at a time, stirring well after each addition until the mixture becomes thick and shiny.

Sift together the all-purpose flour, baking powder, and table salt. Gradually fold the dry ingredients into the wet mixture, stirring gently until just combined. Avoid over-mixing to maintain a tender, fudgy texture.

Pour the batter into the prepared baking pan and spread it evenly with a spatula. Bake for 20 to 25 minutes, or until the edges begin to pull away from the sides of the pan and a toothpick inserted into the center comes out with moist crumbs.

Immediately after removing the brownies from the oven, evenly distribute the miniature marshmallows, chopped almonds, and semi-sweet chocolate chips over the surface of the warm brownies.

Adjust the oven setting to broil. Position the pan approximately 8 inches from the broiler element. Broil the brownies for 1 to 2 minutes, or until the marshmallows are lightly toasted and golden brown. Monitor closely to prevent burning.

Remove the brownies from the oven and allow them to cool in the pan for 20 minutes. For cleaner slices, refrigerate the pan for an additional 20 to 30 minutes. Once set, loosen the brownies from the sides using a bench scraper or knife coated with non-stick spray. Lift out using the parchment overhang and cut into squares.