Pulse hazelnuts in a food processor until finely ground. Combine the sugar and water in a pot over medium heat and stir until sugar dissolves completely. Boil the syrup for three minutes, then add in the ground hazelnuts. Turn the heat down to low and simmer for another three minutes, then slowly increase the temperature to medium high. Just before it starts to boil, remove it from the heat and cover with a lid.
Allow the covered nut mixture to infuse for at least 3 hours or up to 8 hours. Then, strain it through two layers of cheesecloth, discarding the ground hazelnuts for another use. Stir the orange flower water and brandy into the nutty syrup. Use a small funnel to portion the orgeat into bottles or a jar. Store in refrigerator for up to two weeks.