Preheat
the oven to 350°F/180°C/gas 4. Butter a 9-inch
(23-cm) baking pan.
Cookie Base: Sift the flour,
baking powder, and salt into a large bowl. Beat
the butter and brown sugar in a large bowl with an
electric mixer at high speed until creamy.
Add the
vanilla and egg, beating until just blended. Mix
in the dry ingredients until well blended.
Prune Filling: Bring the prunes and water to a boil
in a large saucepan. Reduce the heat and simmer
for 3 minutes. Drain well and transfer the prunes
to a food processor or blender.
Add the honey and
lemon zest and juice and process until smooth. Firmly press one third of the cookie base into the
prepared pan to form a smooth, even layer.
Spoon
over half of the prune filling and spread it evenly.
Sprinkle with half the remaining cookie base and
spread with the remaining prune filling.
Sprinkle
with the remaining cookie base to finish. Bake for
55-65
minutes, or until lightly browned. Cool completely
before cutting into squares.