Step 1: Prepare the Crust

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish or line it with parchment paper for easy removal.
2. Combine Dry Ingredients: In a medium mixing bowl, combine the flour, granulated sugar, and finely ground pistachios. Stir to mix the ingredients evenly.
3. Add Melted Butter: Pour in the melted butter and stir until the mixture forms a crumbly dough.
4. Press the Crust: Press the dough evenly into the bottom of the prepared baking dish. Use the back of a spoon or your fingers to flatten it into an even layer.
5. Bake: Bake the crust for 10 minutes, then remove from the oven and set aside to cool slightly while you prepare the filling.

Step 2: Make the Gooey Butter Filling

1. Beat Cream Cheese and Pistachio Paste: In a large mixing bowl, beat the softened cream cheese and pistachio paste (or ground pistachios) until smooth and creamy. This step ensures that the filling will have a uniform pistachio flavor.
2. Add Eggs and Vanilla: Beat in the eggs, one at a time, until fully incorporated. Then add the heavy cream, vanilla extract, and almond extract, if using.
3. Add Powdered Sugar and Butter: Gradually add the powdered sugar, mixing until smooth. Then, pour in the melted butter and continue mixing until fully combined and creamy.
4. Pour Over Crust: Pour the filling over the partially baked crust, spreading it evenly to the edges of the pan.

Step 3: Bake the Cake

1. Bake: Place the pan in the preheated oven and bake for 35-40 minutes, or until the edges are set and slightly golden but the center still looks gooey. Avoid overbaking, as you want the center to remain soft and gooey.
2. Cool: Remove the cake from the oven and let it cool in the pan for at least 30 minutes. The cake will continue to set as it cools.

Step 4: Make the Whipped Cream Topping

1. Whip Cream: In a medium mixing bowl, beat the heavy cream with an electric mixer on medium speed until soft peaks form.
2. Add Powdered Sugar and Vanilla: Add the powdered sugar and vanilla extract, then continue beating until stiff peaks form. Be careful not to overbeat, as it can turn the cream grainy.
3. Spread on Cake: Once the cake has cooled, spread a thick layer of whipped cream on top.

Step 5: Garnish and Serve

1. Sprinkle with Pistachios: Sprinkle chopped pistachios on top of the whipped cream for extra texture and a pop of color.
2. Slice and Serve: Cut the cake into squares or slices and enjoy! Serve at room temperature or slightly chilled.