Place a rack in the centre of your oven and preheat oven to 325ºF.
Line a 9×13 baking pan with parchment paper, leaving an overhang on two opposite sides.
In a medium bowl, combine the flour, almond flour, baking powder and salt, stir and set aside.
In the bowl of your mixer, beat on medium speed the butter and the sugar, until the mixture is creamy.
Add the marzipan and beat until the marzipan has been incorporated into the butter and sugar mixture and no lumps remain.
Add the egg and the almond extract and then beat until the batter is light and fluffy.
Beating on low speed, add the flour mixture and beat until incorporated to the batter.
Evenly press the dough into the bottom of the lined pan.
Using a pastry brush, brush the cream (or milk) on top of the dough.
Sprinkle the dough with the toasted slivered almonds and lightly press them into the top.
Bake for 18 min, or until just barely beginning to colour around the edges. The top should be dry to the touch. Over-baking these will yield crunchy cookies.
Let them cool in the pan for 5 minutes and then using the overhang of the parchment paper, lift them out of the pan and put them on a wire rack to cool.
Once cool, cut with a sharp knife into 18 pieces or more.
If desired, dust them with a bit of powdered sugar.
Toast the slivered almonds in a 350ºF oven for about 10 minutes or until they begin to colour. Make sure to stir them after the initial 5 min. Put them back in the oven making sure to keep a watch on them. Once golden, remove from the oven and let them cool.
Using parchment paper to line the baking pan and leaving an overhang on opposite ends makes it easier to lift the bars out of the pan for cutting into even portions.