Heat oven to 350 degrees.

Lightly grease a 9-by-13-inch metal or glass baking pan with olive oil and line with parchment paper, leaving an overhang on two sides.

In a medium bowl, combine the flour, brown sugar, cardamon and salt. Add the melted butter and stir with a fork until evenly moistened and various crumb sizes form. Refrigerate while you prepare the cake batter.

In a large bowl, rub the granulated sugar and tea leaves together with your fingertips until the mixture is fragrant. Add the baking powder, baking soda and salt and whisk to combine. Add the sour cream, olive oil, eggs and vanilla, and whisk until very smooth.

Add the flour and stir gently until just combined (do not overmix, a few small lumps are fine).

Transfer the batter to the prepared baking pan and spread in an even layer. Crumble the streusel evenly over the top.

Bake until the streusel is browned and a toothpick inserted into the center of the cake comes out clean, 40 to 50 minutes.

Transfer the cake in the pan to a rack to cool for at least 45 minutes, then run a knife around the edges of the cake and transfer it to a cutting board using the parchment paper overhang. Cut into squares and serve. (You can also serve the cake directly from the pan.) The cake will keep, well wrapped or in an airtight container at room temperature, for up to 4 days. Alternatively, freeze the cake for up to 3 months.