Make the Raspberry Coulis: In a small saucepan, combine the raspberries, sugar and lemon juice. Place over medium heat and bring to a simmer, stirring often to encourage the berries to breakdown and release their juice. Simmer for 5 minutes until slightly reduced, remove from heat and set aside to cool. For a smooth sauce, strain the finished coulis through a fine mesh sieve. The coulis can be made ahead and kept refrigerated for up to 1 week.
Make the Tiramisu: In a large bowl, combine the egg yolks and sugar and whisk until pale and fluffy - about 3 minutes. Fold in the mascarpone and 125ml of Baileys until fully combined and smooth.
Whip the cream to soft peaks and fold through the mascarpone mixture.
In a separate bowl, whisk the egg whites to firm peaks. Gently fold the egg whites through the mascarpone mixture in several additions, taking care to keep the mixture as light and airy as possible. Cover and refrigerate until ready to assemble.
Combine the coffee and remaining 25ml Baileys in a shallow bowl. Briefly dip the biscuits in the coffee mixture and lay an even layer across the bottom of your serving dish. Spread ⅓ of the mascarpone mixture over the biscuits and dust with a little cocoa powder. Arrange another layer of soaked biscuits, followed by a generous layer of raspberry coulis, and another layer of biscuits. Repeat with remaining biscuits and mascarpone mixture, reserving the final dusting of cocoa powder until ready to serve. Refrigerate for at least 4 hours or overnight.
To serve: When ready to serve, liberally dust the tiramisu with cocoa powder, sprinkle with some crushed biscoff and serve alongside the raspberry coulis.