Place the half of the icing sugar and the egg yolks in a medium-sized bowl.
Place the bowl over a saucepan of gently simmering water. Ensure that the water does not touch the bottom of the bowl.
Whisk the icing sugar and egg yolks vigorously for approximately 3 minutes, or until they have doubled in size and the mixture turns foamy and light. I suggest using a hand mixer for this step.
Remove the bowl from the heat, continue whisking the egg yolk mixture for a few more minutes, and let it cool down to room temperature.
Add the mascarpone cheese, vanilla paste, and the rest of the strawberry liqueur to the egg yolk mixture.
Gently fold it in using a hand whisk, avoiding the use of an electric hand mixer to prevent over-whipping the mascarpone cheese. Set the mixture aside.
In a different bowl, whisk the egg whites with the remaining half of the icing sugar until they form stiff peaks. Ensure the bowl is dry and clean, with no trace of egg yolk in the whites.
Combine the stiffened egg whites and ¾ of chopped strawberries with the mascarpone mixture. Use a silicone spatula to gently fold the ingredients from the bottom to the top.
Mix slowly until you achieve a smooth and creamy consistency.