Melt the white chocolate, either in a bain marie over a pan of hot water, or in short bursts in the microwave, stirring often. Do not let the chocolate get too hot.
Gradually whisk in the yogurt, a spoonful at a time. Whisk in the pumpkin puree, lemon juice, sugar, pumpkin spice and vanilla extract.
Whisk the cream in a large bowl with an electric mixer until it forms soft peaks.
Gently fold the pumpkin mixture into the cream, try not to knock out the air. Place the bowl in the fridge while you prepare the next step of the recipe.
Mix the coffee and alcohol in a wide, shallow bowl.
Dunk the ladyfingers into the coffee mixture very briefly, turning them to coat. Arrange them in an approx 25 x 20 x 7cm (10 x 8 x 2 ¾ in) dish as you go, placing them close together to make one complete layer. You can cut some into smaller pieces as needed to fill in any gaps.
Spread half of the pumpkin cream over the top of the layer of ladyfingers.
Dip the remaining ladyfingers into the coffee mixture one-by-one, arranging them on top of the cream layer as you go.
Spread over the other half of the pumpkin cream then cover and refrigerate for at least 8 hours.
Just before serving, use a fine mesh sieve to dust the top of the tiramisu generously with cocoa powder.