For the almond filling:

1. In a medium-sized bowl, beat the softened butter with sugar. Add the eggs, one at a time, whisking vigorously until pale and fluffy. Sift the almond flour, add the vanilla extract and rum (optional), and stir to combine.
2. Refrigerate for 30 minutes. The mixture must be creamy but not too soft or liquid.
3. Preheat the oven to 350°F (180°C) and line a cookie sheet with parchment paper.

For the syrup:

1. In a small saucepan, combine water, sugar and rum. Bring to a simmer for a minute and stir until the sugar dissolves. Remove from heat and let cool to room temperature.

For assembling:

1. Slice each croissant horizontally like you would for a sandwich and arrange on a baking dish, cut side up. Spread about 2 tablespoons of almond filling on the bottom half of each croissant. Place the top halves, brush with the syrup and sprinkle with slivered almonds.
2. Bake on the center rack for about 15 to 18 minutes, or until golden. Transfer to a wire rack and cool to room temperature or just warm. Dust with powdered sugar before serving.