Preheat the oven to 400 degrees F (200 degrees C). Grease a 9-inch round pan.

Beat 1 1/4 cups sugar and butter together using an electric mixer until well blended, light, and fluffy. Add eggs, one at a time, followed by vanilla extract, mixing well after each addition.

Turn mixer to low speed and slowly add in flour, baking powder, and salt, mixing just until combined. Fold in rhubarb.

Pour batter into the prepared baking pan. Smooth the top and place apple slices in a circular pattern on batter. Sprinkle almonds and additional sugar on top.

Bake in the preheated oven until a toothpick inserted into the center comes out clean, 35 to 45 minutes. Let cool, and serve warm or cold.