1) Preheat the oven to 190C/Gas 5.
2) In a large bowl, with an electric mixer, cream the butter and sugar until fluffy. Add the eggs one at a time, and the vanilla and almond essences, and blend well.
3) In a separate bowl, combine the flour, baking powder, salt and ground almonds. Add the flour mixture to the sugar mixture and blend well. Fold in the slivered almonds.
4) Roll out the dough onto a flour-dusted work surface and cut into the desired shapes. Place the biscuits onto a parchment-lined sheet tray and bake for 8 to 10 minutes. Transfer to a wire rack to cool.