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1
Preheat oven to 350 degrees F (175 degrees C).
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2
Stir blackberries, 2 tablespoons sugar, and vanilla extract together in a bowl; let rest, 15 to 20 minutes.
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3
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.
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4
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.
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5
Scatter blackberries over the batter and spoon any accumulated blackberry juices over the top.
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6
Bake buckle in the preheated oven until crisped and browned around the edges, about 1 hour.
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7
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.
