Preheat the oven to 350°F [180°C]. Line a 12-well standard or 6-well jumbo muffin pan with paper liners or coat thoroughly with nonstick cooking spray.
To make the streusel: In the bowl of a food processor fitted with the metal blade, pulse the butter, flour, and brown sugar in 1-second intervals until the butter is cut into pea-size pieces. (Alternatively, combine the butter, flour, and brown sugar in a bowl and use two knives or rub between your fingers to break up the butter into pea-size pieces.) Stir in the raw sliced almonds. The streusel should be a mixture of crumbly and chunky. Set aside.
To make the batter: In a large bowl, whisk together the melted butter and the eggs. Add the milk, almond extract, and vanilla and whisk until well combined. In a medium bowl, whisk together the flour, granulated sugar, baking powder, and salt. Stir in the toasted almonds.
Add the flour mixture to the butter mixture and use a rubber spatula to carefully fold together until almost combined. Gently fold the plums into the batter. Be careful not to overmix, or your muffins will be tough; the batter should still have a couple of streaks of flour.
Divide the batter equally among the prepared muffin wells and mound each well with an equal amount of the streusel.
Bake until the tops are golden brown and a muffin bounces back when you poke it gently in the center with a finger, 22 to 26 minutes for standard size or 30 to 35 minutes for jumbo.
Remove the muffins from the oven and let cool in the pan for 5 to 10 minutes. Carefully lift the muffins from the pan and transfer them to a wire rack to cool a little more. (Use a butter knife to lift the muffins out if you didn’t use paper liners.) Serve warm.