Preheat the oven to 350°F (175°C) and grease a 9×9 inch baking pan.
In a large mixing bowl, cream together the butter and sugar until light and fluffy.
Beat in the egg and almond extract until well combined.
In a separate bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the chopped rhubarb until evenly distributed in the batter.
Spread the batter into the prepared baking pan and sprinkle the sliced almonds on top.
Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.
Dust the cooled cake with powdered sugar before slicing into squares.