Grind the almonds in a food processor until very fine crumbs. Transfer to a bowl and pour over half of the almond extract. Stir to combine.
Cream together the butter and sugar in a large bowl, using an electric mixer on medium-high speed, until light, fluffy and pale yellow, about 3 minutes. Beat in the egg just until incorporated. Mix in the flour, ground almonds and the remaining almond extract on low speed. The dough should be quite stiff.
Press the dough into shortbread molds, filling two-thirds full. Bake, in batches if needed, until golden brown and set, 10 to 12 minutes. Cool in the pans for about 10 minutes, and then gently remove from the molds.