Preheat the oven to 350°F (180°C) and butter and line with parchment a 9-inch (23-cm) springform pan. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment or in a medium bowl with a handheld electric mixer, cream sugar and room-temperature butter until light and fluffy.
Then add in the ricotta and beat on medium-high speed for another 2 minutes or so, until well combined and aerated again.
Add the eggs, one at a time, then add the lemon zest and juice and the almond extract. Beat for another 5 minutes, scraping down the sides occasionally.
In a small bowl, whisk together the flour, baking soda, and salt, then fold into the wet ingredients.
Spread evenly in the prepared pan.
Arrange the plums on top of the cake, covering the whole surface.
Bake for about 55 minutes, until a skewer inserted in the center comes out clean.
Let cool for 15 minutes in the pan before releasing the sides to let cool completely.
Just before serving, garnish with a dusting of powdered sugar, if desired.
Store in an airtight container at room temperature for up to 2 days.