In the bowl of a stand mixer fitted with the whisk attachment, combine the heavy cream, mascarpone, powdered sugar, and vanilla. Whip on medium speed until a thick, fluffy cream forms. Do not overwhip—stop once the mixture holds soft peaks and looks smooth. Set aside.
Pour the milk into a wide, shallow bowl. Quickly dip half of the ladyfingers, one at a time, and arrange them in a single layer in a 12 x 16-inch/30 x 40 cm rectangular baking dish.
Spread half of the mascarpone cream over the ladyfingers and smooth the top using a small offset spatula or the back of a spoon. Scatter the diced strawberries evenly over the cream.
Dip the remaining ladyfingers in milk and arrange them in a second layer over the strawberries.
Spoon the rest of the cream on top and smooth the surface. Cover the dish and refrigerate overnight to allow the tiramisu to set.
Before serving, decorate the top with halved strawberries. Serve chilled, slicing with a large knife and using a cake server or offset spatula to lift the portions.