Prepare the streusel: I a medium bowl, whisk together 3/4 cup of the flour, 2/3 cup of the brown sugar, and 1/2 teaspoon of the salt.
Drizzle the 5 tablespoons of melted butter over the flour mixture and use a fork to distribute it evenly. Add the almonds and toss to combine. Squeeze the mixture into large clumps. Add 3/4 cup raspberries.
Prepare the cake: Preheat the oven to 350°F. Butter an 8-inch square baking dish and line it with parchment paper with a 2-inch overhang on two sides. In a medium bowl, whisk together the remaining 1 1/2 cups flour, the remaining 1/2 teaspoon salt, and 1 1/2 teaspoons baking powder.
In a large bowl, beat the remaining 10 tablespoons butter with the granulated sugar with an electric mixer on medium until fluffy, about 3 minutes. Beat in the egg and the vanilla.
Add half of the flour mixture and beat to combine. Beat in the buttermilk. Then beat in the remaining flour mixture. Fold in the remaining 1 cup raspberries.
Transfer the batter to the prepared pan and smooth the top. Top with the crumb mixture. Bake the cake until a toothpick inserted into the center comes out with moist crumbs attached.
Transfer the cake to a rack to cool completely. Use the parchment overhang to lift the cake out of the pan and transfer it to a cutting board or serving plate. Cut into 16 pieces to serve.