Praliné:
Preheat the oven to 150°C. Place almonds and hazelnuts on a silicone mat or parchment paper and bake for 15 minutes. Allow to cool, then rub between hands to remove most of the skins.
In a heavy saucepan, melt the granulated sugar over medium heat until amber in color. Pour onto a silicone mat or parchment paper to cool completely, then break into pieces.
Blend the nuts and caramel pieces in a food processor until it forms a smooth, liquid praliné. Store in an airtight container in the refrigerator.
Biscuits:
Preheat oven to 160°C.
Cream together butter, muscovado sugar, and vanilla sugar until light and fluffy. Warm the milk slightly and mix with the praliné. Combine with the butter mixture.
Gradually mix in the flour until well incorporated. Transfer the dough to a piping bag fitted with a 6B star nozzle.
Pipe the dough onto parchment paper in the desired shapes.
Bake for 17 minutes or until lightly golden. Allow to cool on a wire rack.