1-Preheat: Start by preheating your oven to 350°F (175°C) and lining a 9×9-inch baking pan with parchment paper for easy removal later.

2-Mix dry ingredients: In a medium bowl, whisk together the flour, cocoa powder, and salt to ensure even distribution of flavors.

3-Prepare batter: Next, in a mixer, beat the softened butter and sugar until fluffy, which usually takes 2-3 minutes, then mix in the vanilla extract and egg yolk. Gradually add the dry ingredients in two batches on low speed until just combined, being careful not to overmix.

4-Bake crust: Press this mixture evenly into the prepared pan and bake for 23-25 minutes, or until the top is dry and a toothpick comes out clean. Let it cool completely before moving on.

5-Preparing the Ganache Filling: For the ganache, heat the heavy cream until it’s very steamy but not boiling, then pour it over the chopped bittersweet chocolate. Cover and let it sit for 5 minutes to melt properly. Afterward, whisk until smooth and stir in the cubed butter for a silky texture. Pour the ganache over the cooled crust and smooth it out evenly.

6-Add toppings and set: Add the topping by sprinkling flaky sea salt if desired, then evenly distribute the mini marshmallows and chopped almonds, pressing them gently into the ganache. If you like, drizzle melted dark chocolate over the top for extra flair. Let the bars set at room temperature for 4-6 hours or refrigerate for 1-2 hours to firm up before slicing.