Line a 9" by 13" baking tray with parchment paper. Set aside.
Remove 3 eggs from the fridge. Separate the egg whites and egg yolks, ensuring the egg whites are added to a clean mixing bowl. Leave at room temperature.
Preheat oven to 375°F/191°C.
Fit the mixing bowl with the egg whites to a stand mixer with a whisk attachment. Gradually whisk on low speed until foamy. Add in the cream of tartar. Increase the speed gradually and add in the sugar in small increments.
Beat until the egg whites reach medium peaks (when the whisk it pulled out, there will be a slight curve to its end). Set aside.
Add the egg yolks to a large bowl. Add in the sugar and whisk together.
Next, add in the oil and milk and whisk until combined.
Add in the sifted cake flour and hojicha powder. Use a spatula or whisk to combine together.
Take ⅓ of the whipped egg whites and fold it into the batter. It's OK to be a little heavy-handed for this first fold since we will add more of the egg whites later.
Next, add another ⅓rd and continue to fold. Repeat with the remainder of the egg whites, until the batter is light and homogeneous.
Transfer the cake batter to a 9" by 13" baking tray and smooth out the top with an offset spatula. Tap the tray a few times to remove any trapped air bubbles.
Place into the oven and bake at 375°F/191°C for 13-15 minutes, until the top is dry and springy to the touch. (Don't overbake or it will be dry).
Remove from the oven and carefully drop it from a height of about 1'.
Transfer the cake over to a cooling rack, with the parchment-side down.
Make the Hojicha Cream:
Add the whipping cream, hojicha powder and sugar to a large bowl. Whisk until the cream thickens.
Tear off a new large piece of parchment paper.
While the cake is still slightly warm, (not too warm or the cream will melt), but not cool so that the cake won't roll.
Carefully flip the cake onto the the new parchment paper, with bottom side facing up. Peel the old parchment paper off the cake.
Using an offset spatula, spread the hojicha cream over the surface of the cake.
Carefully roll up the cake and seal the ends of the parchment. Place into the fridge to chill for 1-2 hours.
Slice with a serrated knife and serve.
Note: Cake can be kept in an airtight container for up to 1 week.