For key visual steps, refer to the body of the post.

Prepare the confetti birthday cake:

• Make the Perfect Swiss Roll Cake according to the recipe instructions (note: only use 50g granulated sugar total) and fold in 2 tablespoon of rainbow confetti sprinkles into the cake batter prior to baking.
• Transfer the cake batter to a 13" by 18" prepared baking sheet lined with parchment paper and smooth the surface with an offset spatula.
• Tap the pan a few times to release any trapped air bubbles.
• Bake the cake for 8-10 minutes at 375°F/191°C, or until the top of the cake is dry and springy to the touch.
• Remove from the oven and let it cool completely on a wire rack.

Make the confetti/birthday cake ice cream:

• Make the vanilla ice cream according to the Matcha Ice Cream Sandwiches recipe.
• Add/stir in 2 tablespoon of confetti sprinkles into the ice cream in the last few minutes of churning.
• Transfer the churned ice cream to a 9" by 13" baking tray lined with parchment paper and use an offset spatula to smooth out the surface.
• Freeze the ice cream for at least 6-8 hours, or until frozen solid.

Assemble:

• Flip the cake upside down so that the parchment layer is on top.
• Gently peel off the parchment paper from the cake.
• Cut the cake in half width-wise, so you end up with 2 x 9" by 13" cakes.
• Place one half of the cake with the browned side facing up onto a piece of parchment paper in a 9" by 13" baking tray or pan.
• Add the slab of frozen confetti birthday cake ice cream onto the cake.
• Layer the second half of the cake with the browned side facing down on top of the ice cream.
• Cover with a lid/plastic wrap and freeze until firm.
• When ready to serve, trim off the edges using a clean, sharp serrated knife and slice into bars or squares to your liking.