• Melt the butter in a saucepan and take it off the heat.
• Blitz the biscuits to fine crumbs in a food processor and stir them into the butter.
• Line the base of the springform tin with baking parchment and tip the biscuit mixture into the tin. Spread it over the whole base and press it halfway up the sides using the bottom of a glass.

  Chill the base while you make the lemon cream.

Lemon cream

• Whisk the egg yolks and condensed milk together for a couple of minutes.
• Add the lemon juice and zest and whisk briefly to combine.
• Pour the cream over the base so that it comes to within a couple of millimetres of the top of the biscuit edge. Use a spoon if the cream needs nudging out to the edges.
• Bake the tart in a preheated conventional oven (top and bottom heat) at 170°C for approx. 20-25 minutes. The cream should have set, so if you gently shake the tin it should be firm.
• Let the tart cool completely on the worktop before putting it in the fridge for at least 3-4 hours, and ideally overnight. Do not release the springform until the tart has chilled and rested.

Mascarpone cream and decoration

• Whisk together the ingredients for the mascarpone cream until smooth and thick.
• Carefully release the tart from the springform tin, peel the baking parchment off the base and place the tart on a serving plate.
• Spread the mascarpone cream over the top and decorate with, for example, grated lemon zest and yellow edible violas.