Step 1: Preheat your oven to 325°F (160°C). In a large bowl, whisk together the egg yolks and 30g of granulated sugar until the mixture is pale and slightly thickened.
Step 2: Gradually whisk in the vegetable oil, milk, and vanilla extract until well combined.
Step 3: Sift the cake flour and matcha powder into the yolk mixture. Gently whisk until the batter is smooth and no green clumps remain.
Step 4: In a separate clean bowl, beat the egg whites with cream of tartar on medium speed until frothy. Gradually add the remaining 70g of sugar, one tablespoon at a time, and continue beating until stiff, glossy peaks form.
Step 5: Gently fold one-third of the egg whites into the matcha batter using a spatula to lighten the texture. Carefully fold in the remaining egg whites in two batches, being careful not to deflate the air.
Step 6: Pour the batter into an ungreased 8-inch chiffon pan or a lined rectangular baking pan. Tap the pan on the counter once to release large air bubbles.
Step 7: Bake for 30-35 minutes or until the cake is springy to the touch. Invert the pan immediately and let the cake cool completely upside down.
Step 8: Once cooled, remove the cake and slice into 12 even wedges or rectangles. Cut a slit in the top of each slice to create a pocket.
Step 9: Whip the heavy cream with powdered sugar until stiff peaks form. Pipe the cream into the cake pockets and garnish with fresh fruit slices. Serve your matcha chiffon cake sandwiches chilled.