plus 3 tablespoons) unsalted butter, at room temperature, plus extra for greasing the pan • granulated or superfine sugar (caster sugar) • Zest of 1 lemon, plus 1 tablespoon lemon juice (or more juice as needed) • vanilla extract (vanilla essence) • almond extract • large eggs • all-purpose flour, sifted • baking powder • salt • almond flour (ground almonds) • fresh blueberries • confectioners' sugar