Line an 8-inch square pan with parchment, leaving overhang on two sides. Roughly chop the almonds and set them aside with the marshmallows. Ensure the mixing bowl and spatula are completely dry.

Place the chocolate and butter in a microwave-safe bowl. Heat at half power in 20–30-second bursts, stirring thoroughly each time, until nearly melted. Stir until the last pieces dissolve; do not keep heating simply because softened chips still hold their shape. Allow up to 5 minutes, depending on your microwave.

Let the chocolate stand until warm rather than hot, stirring occasionally. Fold in the marshmallows and almonds until coated. If the chocolate is hot enough to melt the marshmallows, pause longer before combining.

Scrape into the lined pan and spread evenly, pressing lightly into the corners without crushing the marshmallows. Refrigerate for about 2 hours, until the center is firm. A warm room or deeper pan may need longer.

Lift onto a cutting board using the parchment. Let stand briefly if too hard to cut, then slice into 4 strips and cut each into 5 pieces, making 20 squares. Store covered in the refrigerator for up to 1 week for best quality. Keep cool during transport.