Ensure croissants are dry—ideally left uncovered at room temperature overnight, or lightly bake fresh ones at 175 °F for 30–40 minutes, flipping halfway.

Prepare simple syrup: combine water, rum, and sugar in a saucepan over medium heat until sugar dissolves; set aside to cool.

Make almond filling: in a bowl, blend sugar, almond flour, and softened butter until smooth. Mix in the egg and almond extract until fluffy, with a frosting‑like consistency.

Preheat oven to 350 °F and line a baking sheet with parchment paper.

Quickly dip each croissant half (top and bottom) into the cooled syrup, shaking off excess—don’t let them soak.

Spread 2–3 tablespoons of almond filling onto the bottom half of each croissant; replace the top half, then smooth a thin layer of filling over the top.

Sprinkle with diced almonds. Bake until golden and fragrant.

Cool slightly, dust with powdered sugar, then serve.