Prepare the strawberries for the filling elements. Wash them thoroughly and dice 3/4 of the strawberries into small pieces. Slice the remaining 1/4 of the strawberries to use as the top layer of the filled croissants.
Combine the diced strawberries, granulated sugar, and vanilla extract in a pan. Bring it to a simmer over low heat. Allow the strawberry sauce to reduce for 5-7 minutes, or until the strawberries have softened and coat the back of a spoon.
Add the vanilla mousse to a pastry bag with a decorative tip. Pipe the cream filling on top of the strawberry sauce. If you don’t have a pastry bag, you can spread some of the mousse over the strawberry sauce with a knife instead.