Dry out the croissants by storing them uncovered at room temperature overnight. If fresh, slice horizontally and bake at 175 degrees for 30-40 minutes, flipping halfway, until dry.

In a saucepan over medium heat, combine water, rum, and sugar to create a simple syrup. Heat until the sugar melts and set aside to cool.

In a large bowl, mix granulated sugar, almond flour, and softened butter using a hand mixer until smooth. Add the egg and almond extract, blending until fluffy and smooth.

Preheat the oven to 350 degrees and line a baking sheet with parchment paper.

Dip each side of the croissants in the simple syrup, allowing excess to drip off.

Spread 2-3 tablespoons of almond filling on the inside of each croissant's bottom half, then place the tops on.

Smooth a thin layer of almond filling on top, then sprinkle with diced almonds.

Bake for 15-20 minutes or until golden brown. Dust with powdered sugar before serving.