Lightly grease an 8x8-inch baking pan and line it with foil or parchment, leaving an overhang; set aside.
Place the chopped semi-sweet chocolate in a large heatproof bowl set over a saucepan of barely simmering water (bain-marie), making sure the bowl does not touch the water.
Stir the chocolate frequently until melted and smooth, about 8 minutes.
Add the butter to the melted chocolate and stir until the butter is fully melted and incorporated.
Pour in the sweetened condensed milk and stir until the mixture is smooth and well combined.
Remove the bowl from the heat and stir in the vanilla extract, almond extract, and salt until evenly distributed.
Fold in the slivered almonds and mini marshmallows until they are evenly mixed into the chocolate mixture.
Scrape the fudge into the prepared pan with a sturdy spatula and smooth the top. Refrigerate until firm, at least 2 hours.
Run a butter knife around the pan edges to loosen the fudge, lift it from the pan using the overhanging foil or parchment, peel off the paper, and transfer to a cutting board.
Slice the fudge into pieces and serve or store as desired.