Preheat the oven to 325°F. Lightly spray a 9-inch round cake pan with nonstick cooking spray and line the bottom with parchment paper. Set aside.
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
In a large mixing bowl using a hand mixer on medium-high speed, beat together the almond paste and sugar until small crumbs form.
Add butter and continue to beat the mixture until light and fluffy.
Add the almond extract and the eggs, one at a time, fully combining between each addition.
Add dry ingredients to the wet ingredients and mix until just combined, making sure not to overmix the batter.
Pour the batter into the prepared baking dish, smoothing the top. Sprinkle sliced almonds evenly over the top* and bake for 45-50 minutes, or until a toothpick inserted in the center comes out mostly clean, but a few crumbs are okay.
Allow the cake to cool completely before dusting with confectioners' sugar (if using), cutting, and serving.
Notes
*Be sure to sprinkle the almond slices all the way to the edge. This cake does rise up and the almonds will move towards the center as it bakes.