Preheat oven to 350F.

Prepare Vanilla Cake: Mix flour, sugar, baking powder, powdered milk, and salt with a paddle attachment on a stand mixer on low speed.

Add water, vegetable oil, eggs, and vanilla to the cake mixture. Mix on medium speed until combined, scraping the bowl as needed.

Pour the cake batter into prepared cake pan(s) and bake until a toothpick inserted into the center comes out clean.

Prepare Vanilla Swiss Buttercream: Place egg whites in a sterilized stand-up mixing bowl set over a saucepan with barely simmering water (double boiler).

Heat the egg whites to 160F, stirring constantly to prevent them from turning into an omelet.

Add cream of tartar to the egg whites and whip in a stand mixer until cool and a medium peak forms.

Gradually add unsalted butter, one stick at a time, to the meringue while whipping. Be patient and mix until smooth and well incorporated.

Prepare Caramel Spread: In a saucepan, combine water, sugar, and salt over medium heat.

Bring the mixture to a simmer without stirring until it turns a light golden color.

Continue cooking for about 1 minute more until it reaches a medium golden color.

Immediately add heavy cream and turn the heat to low.

Stir the caramel with a heat-resistant spatula until it reaches 225°F (107°C). Stir in vanilla extract and cool to room temperature.

Prepare Ganache: Place milk chocolate in a bowl.

Place heavy cream in a microwave-safe bowl and heat in the microwave until scalding hot, avoiding overboiling.

Pour the hot cream over the chocolate and stir lightly until the mixture thickens.

Assemble the Cake: Start with a base of vanilla cake.

Create a buttercream barrier around the edge of the cake with a piping bag.

Fill and spread the center with a thin layer of ganache.

Drizzle caramel on top using a piping bag.

Sprinkle crushed shortbread cookies on top of the caramel.

Place another layer of cake and repeat the process.

Crumb coat the cake, chill it, and then apply the final coat of buttercream.

Serve and enjoy the Tim Tam Cake.