Combine the almonds and flour in a food processor and process until finely ground but not oily.
In a bowl, cream the butter and sugar with an electric mixer on medium speed until smooth and fluffy, about 3 minutes. Beat in the egg and lemon zest. Add the almonds and almond extract, and beat, scraping down the sides of the bowl occasionally, until smooth and light.
Use the frangipane immediately or seal airtight and store in the refrigerator for up to 2 weeks.