Chef John's Blackberry Buckle

Preheat oven to 350 degrees F (175 degrees C). Stir blackberries, 2 tablespoons sugar, and vanilla extract together in a bowl; let rest, 15 to 20 minutes. Whisk flour, 3/4 cup sugar, almond meal, baking powder, salt, and cinnamon together in a bowl. Stir milk into flour mixture until batter is just combined. Melt 6 tablespoons butter in a skillet over medium-high heat; pour into the bottom of a 8x12-inch baking dish. Pour batter over melted butter into the baking dish, using a spatula to gently distribute batter without mixing it into the butter, leaving a 1-inch butter border around the sides of the dish. Scatter blackberries over the batter and spoon any accumulated blackberry juices over the top. Bake buckle in the preheated oven until crisped and browned around the edges, about 1 hour. Melt remaining butter in a small skillet over medium-high heat. Cook and stir almonds in hot butter until golden brown and toasted, 2 to 3 minutes. Sprinkle toasted almonds over blackberry buckle and cool buckle to room temperature.