Preheat the oven to 350ºF. Lightly grease a 9-inch round cake pan. Set aside.

Melt 4 tablespoons (57g) of butter in a small pot over medium heat. Once melted, add ½ cup (100g) of light brown sugar. Stir to combine. Cook over medium heat for 3 minutes, stirring occasionally, until the sugar dissolves and the mixture darkens slightly in color.

Turn the heat to low and add 2 tablespoons (30g) heavy cream, 1 teaspoon orange zest, and 1 teaspoon vanilla extract. Whisk until smooth and simmer for 1 minute. Turn off the heat.

Pour the mixture into the prepared cake pan and smooth it into an even layer. Transfer to the refrigerator for 15 minutes.

Break the oranges into segments and slice each segment in half lengthwise. Refer to Note 1. In a bowl, gently toss the oranges with 1 tablespoon (7g) cornstarch and 1 tablespoon (12g) granulated sugar until evenly coated.

Sift ¾ cup (90g) of all-purpose flour, 1 tablespoon (7g) powdered buttermilk, 1 teaspoon baking powder, and ¼ teaspoon kosher salt into a small bowl. Set aside.

Add 4 tablespoons (57g) of room temperature butter and ½ cup (100g) of granulated sugar to a large bowl. Use an electric hand mixer to cream them together. Add 2 eggs and 2 tablespoons (32g) pistachio butter and beat until smooth.

Add half the sifted dry ingredients and beat with the mixer until just combined. Add half the milk and beat until just combined. Add the remaining dry ingredients and the remaining milk, along with 1 teaspoon orange zest and 1 teaspoon vanilla extract. Beat until just combined. Don’t over-mix!

Remove the cake pan from the refrigerator. Arrange the oranges in tight, concentric circles, overlapping each segment slightly to create a rose shape. Pour the batter over the oranges and smooth it out with the back of your spatula.

Transfer the cake to the oven and bake for 48 to 50 minutes until a toothpick inserted into the center of the cake comes out clean.

Let the cake stand at room temperature for 1 hour. The cake should still be warm to the touch.

Place a large plate over the cake pan and carefully invert the cake onto the plate. Let stand for 1 hour until completely cool.

Combine 1 cup (240ml) heavy cream, 1 teaspoon orange zest, and 1 teaspoon vanilla extract in a mixing bowl. Sift ½ cup (60g) powdered sugar into the bowl. Beat with an electric mixer until soft peaks form.

Add 4 ounces (112g) mascarpone cheese to the bowl and beat until stiff peaks form. Be careful not to overmix. Stop mixing as soon as the mixture tightens up into a stiff, spreadable frosting, otherwise you’ll risk breaking the mixture.

Cover and refrigerate the frosting until the cake cools completely.

Transfer the frosting to a piping bag and frost the top of the upside-down cake, or use a rubber spatula to pile the frosting on top and smooth it out.

Sprinkle crushed pistachios on top. Slice the cake and enjoy!