Mix together the brown sugar, espresso and cinnamon in a small bowl and set aside.
Whisk sugar, salt flour and spices together in a bowl.
In a small saucepan, brown the butter. Cook it on medium and stir frequently, until you get small brown flecks at the bottom of the pan. As soon as you get those flecks, take it off the heat and pour over the filling mixture. Whisk it until it comes together, then keep it in the fridge.
Preheat oven to 350°F/175°C. Grease a 8x8" pan with butter and parchment paper.
In a stand mixer, beat the butter, salt, sugars, baking powder and vanilla until smooth with a paddle attachment. Add the eggs one at a time, beating well after each addition.
In a jug, whisk the sour cream and milk together until combined.
Add ⅓ of flour to the stand mixer bowl, beating until well combined. Add ⅓ of the milk mixture. Repeat until everything is combined.
Pour half of the batter into the prepared pan. Sprinkle the filling on the batter, then pour the remaining batter on top. Sprinkle the streusel topping over the batter.
Bake until golden brown, 30-35 minutes. A toothpick inserted will come out clean
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. I have provided a conversion to US cups but please note that I haven’t tested the recipe with this method. Just click on ‘US customary’ above the ingredients.Room Temperature: I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients about 2 hours before you start baking. Salt: If you don't have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt.Butter: I use European style butter, like Kerry Gold, in my recipes. This will make your finished bake much creamier and richer.Storage: Keep the cake wrapped up tightly and refrigerate for up to 2 days.