To make the orgeat syrup: heat oven to 350 F.
Spread the almonds in a single layer on a rimmed baking sheet. Bake for 5 to 7 minutes, tossing halfway through, until light golden brown.
Immediately transfer the almonds to a food processor and whirl for 10 seconds, or until coarsely ground.
In a small pot, heat the sugar and water over medium heat, stirring occasionally, until the sugar dissolves and the mixture simmers. Stir in the ground almonds and return to a simmer. Remove from heat and cover. Let sit at room temperature for 3 hours or up to 8.
Strain the syrup through a double layer of cheesecloth into a pint jar. Stir in the brandy and orange blossom water. Chill until ready to use. Note: the syrup can be stored in the refrigerator for up to 2 weeks.
To make the Mai Tai: Fill a shaker halfway with ice cubes. Add the rums, lime juice, Grand Marnier, orgeat syrup and a dash of bitters. Shake vigorously until the outside of the shaker is frosty.
Fill a rocks glass with crushed ice and strain the Mai Tai over it. Garnish with a mint sprig, lime wheel and cherry. Enjoy!