1. Preheat the oven to 350°F / 175°C. Generously butter a 7" springform pan. Set aside.
2. Heat 75g of milk in the microwave in 30-second intervals until it begins to bubble, then place an earl grey tea sachet in the warm milk. Set it aside to steep.

Earl Grey Streusel

1. In a medium sized bowl, melt the butter. To the butter, add the sugar, flour, and the contents of an earl grey tea sachet. Use a fork to mix the ingredients together until you have formed clumps of dough. Set aside.

Butter Cake

1. In a small bowl, whisk together the flour, baking powder, and salt. Set aside.
2. In the bowl of a stand mixer fitted with the paddle attachment (or with a hand mixer, in a large bowl) beat together the butter and sugar for 3-4 minutes, until light and fluffy.
3. Use a rubber spatula to scrape down the sides of the bowl. Add in the egg and the vanilla. Beat until well combined (~1 min.)
4. Add in half of the flour mixture, and mix by hand just until the flour is incorporated. Remove the tea sachet from the milk, squeezing the sachet to remove the milk from it. Add the milk and sour cream into the batter and mix until combined. Add in the rest of the flour, and lightly fold the mixture together just until all of the flour is incorporated.
5. Pour the cake batter into the prepared cake pan, smoothing out the top. Gently place the halved blackberries in an even layer over the cake, then top it with the earl grey streusel.
6. Bake for 45-50 minutes, until a toothpick inserted in the center of the cake comes out clean or with only a few moist crumbs.
7. Let the cake come to room temperature, then slice, serve and enjoy!