Preheat your oven to 350°F (175°C). In a mixing bowl, combine all-purpose flour, chilled unsalted butter, powdered sugar, and egg yolk until a crumbly dough forms. Gradually add ice water until the dough comes together. Roll it out and press into tartlet pans, ensuring even coverage. Bake for 15-20 minutes or until golden brown. Allow the tartlet shells to cool completely on a wire rack.

While the tartlet shells cool, prepare the coffee ganache. In a saucepan, gently heat heavy cream until it’s just simmering. Remove from heat and add bittersweet chocolate and brewed coffee. Stir continuously until the mixture is smooth and glossy. Transfer the ganache into a bowl and refrigerate for about 30 minutes, or until it’s firm enough to hold its shape when piped.

In a separate bowl, combine mascarpone cheese, granulated sugar, and vanilla extract. Using an electric mixer, whip the mixture on medium speed until perfectly smooth and creamy, about 2-3 minutes. Be cautious not to overmix. Set the mascarpone cream aside in the refrigerator.

Once the tartlet shells have cooled and the ganache is set, fill each shell with a generous dollop of the coffee ganache. Layer the mascarpone cream on top, filling each tartlet to the rim. Smooth out the tops with a spatula.

Place the assembled Tiramisu Tartlets in the refrigerator for at least 30 minutes. Before serving, dust with unsweetened cocoa powder. Optionally, top with chocolate shavings or fresh berries.