1. Begin by rinsing 2 cups of sushi rice in cold water until the water runs clear to remove excess starch.
2. Cook the sushi rice in 2 1/2 cups of water using a rice cooker or pot according to package instructions.
3. Once cooked, while the rice is still warm, stir in 1/4 cup rice vinegar, 2 tablespoons of sugar, and a pinch of salt. Allow it to cool to room temperature.
4. Line a springform cake tin with plastic wrap to make the cake easy to remove.
5. Slice 2 ripe avocados thinly, cut 2 nori sheets into desired shapes, and prepare any toppings such as sesame seeds, cucumber slices, or fish roe.
6. Begin assembling the avocado sushi cake by creating the first layer in the springform pan with an even spread of sushi rice. Press it down gently with a spatula or spoon.
7. Add thin slices of avocado and a layer of nori on top of the rice.
8. Place a layer of thinly sliced roast beef over the avocado and nori.
9. Continue layering with additional layers of sushi rice, avocado slices, and ingredients as desired, pressing each layer gently to maintain consistency.
10. Decorate the top of the sushi cake by arranging avocado slices in a fan-like pattern and sprinkling sesame seeds, fish roe, or scallions for visual appeal.
11. Refrigerate the assembled sushi cake for at least 30 minutes to allow the layers to firm up and flavors to meld together.
12. Carefully remove the springform pan or plastic wrap and transfer the cake onto a serving platter.
13. Slice the avocado sushi cake like a traditional cake using a sharp knife.
14. Serve with soy sauce, wasabi, and pickled ginger on the side for dipping and additional flavor.