1. In a large bowl or stand mixer cream the butter and sugar together until light and fluffy. Scrape down the sides of the bowl from time to time so the all the ingredients are thoroughly mixed.
2. Add the eggs and vanilla extract and beat until combined.
3. Mix the flour, baking powder and oatmeal together. Gradually add to the creamed mixture and fold in until you have a stiff dough. Remove from the bowl and knead gently. Wrap in cling film and leave to rest in the fridge for a few hours or overnight.
4. Line 2 baking trays with parchment paper. Roll the dough between 2 sheets of baking parchment until its about 5mm thick. If its too soft, place the rolled dough in the fridge and chill until its firm.
5. Cut the dough into 4 rectangles 20cm x 15cm and place on to the prepared trays. Prick the surface all over with a fork then place in the fridge for 30 minutes.
6. Heat your oven to 170°C/Gas 3. Bake the biscuits for 20-25 minutes until the biscuit is golden and crisp. Leave to cool on the tray for 10 minutes then finish cooling on a wire rack.
7. To make the filling, beat the butter until soft and gradually add the icing sugar to form a thick buttercream. Add the melted chocolate and beat again until smooth.
8. To assemble, place one rectangle biscuit on the board, using a palette knife cover the surface with a thin layer of buttercream. Place the second biscuit on top and press down gently, repeat by covering with buttercream.
9. Continue sandwiching the torta together with the remaining biscuit and buttercream leaving the top layer uncovered.
10. Wrap the whole cake in parchment paper then foil and leave for 3-4 days. The longer the cake is left the softer the biscuit becomes.
11. Before serving, unwrap and dust the top with plenty of icing sugar. Slice into fingers and enjoy with a cup of tea.