In a medium saucepan, combine chopped strawberries, lemon juice, erythritol, and water. Cook over medium heat for 5 minutes until the strawberries soften and release their juices. Stir occasionally to prevent sticking. Once done, remove from heat and allow the mixture to cool completely.

Preheat your oven to 180°C (350°F). Prepare an 8-inch square baking tin by lining it with parchment paper.

In a large mixing bowl, whisk together softened butter and erythritol until light and creamy. Add egg yolks and vanilla extract, mixing until fully incorporated. Gradually incorporate coconut flour, salt, and baking powder, stirring until smooth.

In a separate clean bowl, beat egg whites until stiff peaks form, then gently fold into the butter mixture.

Pour the prepared batter into your lined baking tin, spreading it evenly. Bake in the preheated oven for about 25 minutes or until the cake is golden brown and firm to the touch.

In a mixing bowl, beat together mascarpone and whipping cream until smooth. Slowly add erythritol and vanilla extract, mixing until fluffy.

In a baking dish, layer cooled strawberry sauce, chopped sponge cake, and creamy mascarpone mixture. Repeat until the dish is full, finishing with a cream layer topped with fresh strawberry slices.

Cover your assembled Keto Strawberry Tiramisu and refrigerate for at least 1 hour.