1-First, start by making the syrup. Combine 1 cup water, 2 tablespoons granulated sugar, and 4 tablespoons rum (or 1 teaspoon vanilla extract) in a small saucepan. Bring it to a simmer for one minute, stirring until the sugar dissolves, then remove from heat and let it cool completely.

2-Next, prepare the almond cream. If using whole almonds, pulse 1/2 cup granulated sugar, 2/3 cup whole blanched almonds, and 1/8 teaspoon salt in a food processor until finely ground. Add the 1 stick (8 tablespoons) sliced unsalted butter and mix well, then add 2 large eggs one at a time, processing until creamy. If using almond meal or flour, combine 1/2 cup granulated sugar, 1 cup almond meal or flour, and 1/8 teaspoon salt in a stand mixer. Blend in the butter, then add the eggs one at a time until fluffy.

3-Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Slice each of the 8 one-day-old croissants horizontally like a sandwich. Dip each half into the cooled syrup, coating lightly on both sides and ends.

4-Assembling and Baking: Place the croissant halves cut side up on the baking sheet and spread 2 tablespoons of almond cream on each bottom half. Put the tops back on, then spread 1 tablespoon of almond cream over each top and sprinkle with 3 tablespoons sliced almonds. Bake on the center rack for 15 to 18 minutes until the cream turns golden. Let them cool on a wire rack or serve warm, and dust with powdered sugar before eating.