Combine blackberry preserves and water in a small bowl until smooth.

Add confectioners sugar and whisk to combine.

Adjust with sugar or water to achieve a fluid, yet coating consistency.

Preheat oven to 375°F (190°C).

Coat tea cake pan with nonstick spray.

Melt butter and set aside to cool.

Whisk together cake flour and green tea powder in a separate bowl.

Beat eggs, salt, and granulated sugar with a mixer on medium speed for about 8 minutes until pale yellow and very thick.

Stir in vanilla.

Gently fold in the flour mixture rapidly.

Fold in the melted butter, ensuring it doesn't settle at the bottom.

Quickly spoon mixture into the prepared pan, filling to just below the rim.

Bake for 11-13 minutes, or until cakes spring back when touched lightly.

Invert pan onto a wire rack and flip cakes facing up.

Spoon glaze onto the warm cakes.

Let cool and serve.