Whisk two eggs in a saucepan and add lemon zest, juice, butter, sugar, and cornflour.
Stir over low heat until the curd thickens, then pour into a bowl and chill.
Preheat your oven to 180°C and grease a 12-hole muffin tin with spray or butter.
Line the bottom of each tray hole with circles of baking parchment.
Cream the baking spread and caster sugar in a mixer until smooth and pale.
Mix in three eggs, then fold in flour, salt, baking powder, thyme, and lemon zest.
Fill each muffin hole three-quarters full and bake for 15 to 20 minutes until golden.
Cool cakes in the tin briefly, then transfer to wire racks to cool completely.
Heat caster sugar, thyme, and lemon juice in a saucepan until a syrup forms.
Pierce the sponges using a skewer and brush the warm syrup over the cakes.
Mix remaining syrup with sugar and zest, then drizzle over the tops of the cakes.
Whisk butter and icing sugar together, stir in mascarpone, and fold in whipped cream.
Transfer the mascarpone filling into a piping bag with a round nozzle.
Halve the cakes, pipe cream on the bottom halves, and spread curd on the top halves.
Assemble cakes and garnish with candied lemon slices and thyme leaves.