Preheat the oven to 170°C and line and grease 3 x 9 inch round cake tins.
Beat together the sugar and butter until pale, light and fluffy.
Add the eggs one at a time, beating on a low speed.
Add the vanilla extract, lemon zest and thyme leaves and stir in.
Sift the flour, baking powder, bicarbonate of soda and salt together.
Add the flour alternately with the buttermilk, adding the flour in three additions and the buttermilk in two (begin and end with the flour), scrape down the sides of the bowls as needed and mix until just combined.
Pour the batter equally across the cake tins and bake for about 35 minutes until an inserted cocktail stick comes out clean.
Once the cakes have been removed from the oven and taken carefully out of their tins, brush the limoncello over the top of the sponges then leave to cool.
Pour the double cream into a large saucepan along with the sprig of thyme. Heat until the cream is just starting to think about boiling then remove from the heat. Let the thyme infused cream cool in the fridge until cold (or overnight if you wish).
Remove the thyme from the cream and whisk until the cream forms soft peaks.
Add the mascarpone, icing sugar and limoncello and mix together until combined, the cream will probably stiffen up a little more but be careful not to overmix.