1.Make almond praline. Place nuts on baking paper-lined oven tray. Combine sugar and the water in small frying pan. Stir over heat, without boiling, until sugar is dissolved, then boil, uncovered, without stirring, until golden brown. Pour toffee over nuts; set at room temperature, then process until fine.
2.Next make custard filling. Combine sugar and flour in small saucepan; stir in combined egg yolks and milk until smooth. Stir until mixture boils and thickens. Simmer, stirring, over low heat, 1 minute; remove from heat. Cover surface with plastic wrap; refrigerate until cold. Beat butter and extract until as white as possible; beat in sifted icing sugar. Beat in custard, in batches, until smooth.
3.Preheat oven to 160°C/325°F. Grease oven trays; line with baking paper.
4.Process flour, ground almonds and butter until crumbly. Add egg yolk and extract; pulse until combined.
5.Knead dough on floured surface until smooth; roll until 3mm (⅛ inch) thick.
6.Using 3.5cm (1½ inch) round cutter, cut 72 rounds from dough. Place about 2cm (¾ inch) apart on oven trays. Bake about 12 minutes. Cool on trays.
7.Sandwich cookies with filling. Spread a little more filling around sides, then roll in praline and dust with sifted icing sugar.