Preheat oven to 350F/180C.
In a large bowl bowl cream butter, sugar and orange zest until light and fluffy, add vanilla and mix well.
With a motor of a stand mixer still running add slowly add flour and ground almonds, continue mixing until a smooth dough forms.
Form cookies by rolling little pieces of dough in your hands and then roll them in ground almonds.
Place on a parchment lined cookie sheet and make an indentation in the center of each ball with your thumb or a teaspoon.
Bake for 10 minutes, then take the cookies out of the oven and check if the thumbprints on your cookies have become too shallow. If that is the case, gently push on them with a teaspoon.
Bake for additional 5-7 minutes until the edges of the cookies are slightly coloured.
Meanwhile melt your chopped chocolate with butter and golden syrup in a double boiler or in a bowl set over a pot of simmering water.
Take the cookies out of the oven and fill half of them with chocolate. Sprinkle with sea salt and let cool on a wire rack.