Make the streusel
In a small bowl, combine the brown sugar, oats, almonds, flour, cinnamon and melted butter and mix until crumbly.
Make the tea cake
Preheat the oven to 350° and grease an 8-by-4-inch loaf pan. In a small bowl, whisk the flour with the baking powder and salt.
In a food processor, combine the almonds and granulated sugar and pulse until finely ground. Transfer the ground almonds to a large bowl and add the butter; beat with a handheld electric mixer at medium speed until fluffy, about 3 minutes. Beat in the eggs, one at a time, then the vanilla. At low speed, beat in half the flour mixture and then all of the milk, followed by the remaining flour mixture.
Pour half the batter into the prepared pan and sprinkle with half the streusel mixture. Top with the remaining batter and use a knife to swirl. Sprinkle the remaining streusel on top and bake for 50 minutes to 1 hour, until a toothpick inserted in the center comes out clean. Let the cake cool completely in the pan on a rack. Unmold and dust the top with confectioners’ sugar.