Preheat the oven to 350ºF.
In a food processor, pulse together the almonds with the flour until completely ground. Set aside.
Cream together the softened butter and the sugar in a mixmaster fitted with the paddle attachment. Continue beating until the mixture turns white.
Beat in the lemon zest and rum.
Beat in the egg whites, one at a time until completely incorporated.
Stir the ground almond and flour into the batter by hand.
Transfer the batter to a piping bag or plastic bag and pipe 1-inch mounds onto a parchment paper-lined cookie sheet.
Bake the wafers for 12-15 minutes until golden brown.
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