In a bowl, combine the boiling water and instant espresso powder. Whisk until the espresso powder is completely dissolved.

Add the Kahlua and stir well. Set aside and allow the mixture to cool completely.

Create a double boiler by placing a heatproof bowl over a saucepan of gently simmering water.

Add the egg yolks and sugar to the bowl. Whisk continuously for 8 to 10 minutes.

Halfway through cooking, add the milk to help smooth the mixture.

Continue whisking until the mixture becomes smooth and slightly thickened. Remove from heat and let cool completely.

In a chilled mixing bowl, add the cold heavy whipping cream and vanilla extract.

Using a stand mixer or hand mixer, whip until stiff peaks form.

Be careful not to overwhip, as the cream can turn into butter.

In a large bowl, combine the cooled egg yolk mixture and mascarpone cheese.

Mix until smooth and fully incorporated.

Gently fold the whipped cream into the mascarpone mixture using a spatula. Fold carefully to maintain a light and airy texture.

Quickly dip each ladyfinger into the cooled espresso mixture for just a few seconds.

Arrange the soaked ladyfingers in a single layer across the bottom of a 9x9-inch baking dish.

Spread half of the mascarpone mixture evenly over the ladyfingers.

Repeat with another layer of espresso-soaked ladyfingers.

Spread the remaining mascarpone mixture evenly over the top.

Cover the dish with plastic wrap, foil, or wax paper.

Refrigerate for at least 8 hours before serving.

For the best flavor and texture, allow the tiramisu to rest for up to 2 days.

Before serving, dust the top generously with unsweetened cocoa powder.

Slice and serve chilled.