Preheat your oven to 350 degrees F. Grease a 9-inch springform pan or line the bottom with a circle of parchment paper.
In a large mixing bowl, beat together the egg yolks and sugar until light and fluffy. Add the melted butter, flour, lemon zest, and baking powder and mix until just combined.
Add the ground almonds and mix to combine. Set aside. (Don't worry if the dough is crumbly and thick. It should be at this stage.)
Beat the egg whites in a medium mixing bowl until stiff peaks form. Add the beaten egg whites to the almond dough and mix until evenly incorporated.
Pour the cake batter into the prepared pan. Sprinkle the top with sliced almonds.
Bake for 45 minutes or until a toothpick inserted in the center comes out clean.
Remove from the oven and allow to cool for 10-15 minutes. Run a knife around the edge of the pan before removing the ring from the cake. Cut into slices and serve warm or a room temperature.
Notes
If the almonds on top start to brown too quickly, carefully place some aluminum foil loosely oven the pan until the cake is done baking.