In a small microwave-safe bowl, heat buttermilk on high in 10-second intervals, stirring between each, until warm (120°F/49°C to 130°F/54°C). (Be careful not to overheat buttermilk.) Stir in tea; let stand for 5 minutes. Using a fine-mesh sieve, strain buttermilk mixture into a small glass liquid-measuring cup; discard solids. Add additional buttermilk, if necessary, to yield ⅓ cup (80 grams) buttermilk mixture. Let cool to at least 80°F (26°C) before using.

Preheat oven to 325°F (170°C). Spray a light-colored metal 9-inch springform pan with baking spray with flour. Line bottom of pan with parchment paper.

In a medium bowl, whisk together cooled buttermilk mixture, condensed milk, melted butter, eggs, oil, and vanilla.

In a large bowl, whisk together flour, sugar, baking powder, and salt. Gradually add flour mixture to buttermilk mixture, stirring just until combined. Pour and spread batter into prepared pan, smoothing with an offset spatula. Sprinkle Spiced Streusel on top, clumping and crumbling as desired.

Bake until a wooden pick inserted in center comes out clean, 22 to 30 minutes, rotating pan halfway through baking. Let cool in pan for 10 minutes. Remove from pan. Serve warm or at room temperature. Drizzle with condensed milk just before serving.