Grease 2 baking sheets.
Put the butter and flour in a food processor and blend until the mixture resembles breadcrumbs. Add the sugar, egg yolks and vanilla bean paste and blend to make a smooth dough. Wrap in clingfilm and chill for at least 1 hour.
Roll out the cookie dough on a lightly floured surface. Cut out rounds using a 6 cm (2½ inch) cookie cutter, rerolling the trimmings to make 16 in all. Place on the baking sheets, spaced slightly apart.
Cut out eyes in half the rounds, using a 1 cm (½ inch) cookie cutter or the end of a large, plain piping nozzle. Also cut out a large, smiling mouth, using a small, sharp knife or scalpel. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 15 minutes, or until pale golden, then transfer to a wire rack to cool.
Make the buttercream by beating together the icing sugar, butter and measurement water until smooth and creamy. Spread the buttercream over the plain cookies, then spread with the jam. Gently press the face cookies on top.