Melt the butter in a small saucepan.
Process the biscuits into fine crumbs using a food processor.
Mix the crumbs with melted butter and press into a 25cm loose-bottomed flan tin.
Chill the crust in the fridge until firm.
Whisk the egg yolk and caster sugar together in a bowl.
Beat the mascarpone and vanilla essence into the yolk mixture.
Whisk the egg white in a separate bowl until stiff peaks form.
Gently fold the egg white into the mascarpone mixture.
Spoon the filling onto the biscuit base and smooth the top.
Arrange the sliced strawberries on top and chill in the fridge.
Remove the tart from the fridge 20 minutes before serving.