Preheat the oven to 350°F (conventional) or 325°F (convection). Grease a cake pan or line it with parchment paper.
Zest the orange and place it in a bowl with the sugar. Rub the orange zest into the sugar with your fingers to release its aroma.
Add the butter and vanilla extract and beat until light and fluffy for several minutes using a hand mixer.
Add the eggs gradually, beating until well combined.
Finely grind ¾ cup of pistachios in a mixer or chop them finely.
In a separate bowl, combine the almond flour, ground pistachios, baking powder, and salt.
Gently fold the dry ingredients into the wet ingredients until just combined. Stir in the ricotta until a smooth batter forms.
Pour the batter into the prepared cake pan and bake for about 40-45 minutes. Test for doneness using a toothpick. Once ready, let the cake cool in the pan for a bit before inverting it onto a wire rack to cool completely.
For the frosting, beat the butter in a bowl until creamy. Add the powdered sugar and then beat until well combined and smooth.
Zest the orange again and juice it. Add about 1 teaspoon of orange zest and 2-3 tablespoons of orange juice, along with the ricotta, to the butter-sugar mixture. Beat until smooth.
Once the cake has cooled completely, spread the orange-ricotta frosting evenly on it.
Chop the remaining pistachios and garnish the cake with them.
