In a small bowl, lightly whisk the egg; set aside. In the bowl of a food processor fitted with a metal blade, pulse the toasted almonds and granulated sugar until the almonds are finely ground. Add the flour and salt; pulse until combined. Add the cold butter and pulse until the mixture resembles coarse sand, approximately 6 to 10 quick pulses. Add the whisked egg and pulse just until the dough begins to form a cohesive mass. Remove the dough, shape it into a disk, wrap tightly in plastic wrap, and refrigerate for at least 1 hour or overnight.

Place the chopped white chocolate in a heatproof bowl. In a small saucepan over medium heat, bring the heavy cream just to a boil. Immediately pour the hot cream over the white chocolate. Allow it to stand for 30 seconds, then gently whisk until smooth and fully melted. Cover the bowl and refrigerate for at least 4 hours or overnight until the mixture is firm enough to whip.

Lightly flour a work surface. Divide the chilled dough into six equal portions. Roll each portion into a smooth disk approximately 5 1/2 inches in diameter. Gently press each disk into a 4-inch tart pan with a removable bottom, ensuring the dough is evenly pressed along the sides and bottom. Place the tart pans in the freezer for 30 minutes to firm the dough.

Preheat the oven to 375°F (190°C). Line each tart shell with aluminum foil and fill with pie weights or dried beans. Bake for 15 minutes. Carefully remove the foil and weights, then continue baking for an additional 10 minutes, or until the shells are lightly golden. Transfer to a wire rack and allow to cool completely.

Remove the chilled white chocolate ganache from the refrigerator. Transfer to the bowl of a stand mixer fitted with the whisk attachment. Whip on medium speed until soft peaks form. Avoid overwhipping. Gently fold in the shredded coconut and light rum (if using) until evenly incorporated.

Divide the coconut filling evenly among the cooled tart shells, smoothing the tops. Place three whole toasted almonds on top of each tart. Refrigerate while preparing the chocolate glaze.

Place the chopped milk and dark chocolates in a medium heatproof bowl. In a small saucepan, heat the heavy cream over medium heat until it just begins to boil. Pour the hot cream over the chocolate and whisk until smooth and fully combined. Allow the glaze to cool for approximately 10 minutes to thicken slightly.

Remove the tarts from the refrigerator. Spoon the chocolate glaze evenly over each tart, covering the coconut filling. Return the tarts to the refrigerator and chill until the glaze is fully set, approximately 10 minutes.

The tarts can be stored, tightly covered, in the refrigerator for up to 2 days.