Begin by preheating your oven to 350°F (175°C). Grease an 8x8-inch baking pan and line it with parchment paper for easy removal later. In a medium saucepan over low heat, melt the butter. Once melted, remove from heat and stir in the sugar and cocoa powder until fully combined. This creates a rich, glossy base that smells absolutely divine—like a warm hug in a bowl!

In a separate bowl, whisk the eggs and vanilla extract until frothy. Slowly pour this mixture into the cocoa-butter blend, stirring continuously to ensure everything is well incorporated. The batter will thicken slightly, taking on a velvety texture that promises indulgence in every bite.

Sift the flour, baking powder, and salt into the wet mixture. Fold gently until no streaks of flour remain. Be careful not to overmix; doing so could make the bars dense instead of soft and chewy. Pro tip: Use a spatula to scrape the sides of the bowl to ensure even mixing.

Pour half of the batter into the prepared pan, spreading it evenly with a spatula. Sprinkle half of the marshmallows, nuts, and chocolate chips over the top. Repeat with the remaining batter and toppings, pressing down lightly to secure the layers. For an extra touch, sprinkle a pinch of flaky sea salt on top—it enhances the sweetness beautifully.

Bake the bars for 25-30 minutes, or until the edges are set but the center is still slightly soft. Avoid overbaking, as the residual heat will continue to cook the bars while they cool. Let them rest in the pan for 10 minutes before transferring to a wire rack to cool completely. The aroma wafting through your kitchen will have everyone eagerly waiting to dig in!