To make the batter, mix the unsalted butter and white chocolate in a large bowl. Place the bowl in the microwave or over a double boiler. Microwave for 15 to 30 second increments, stirring in between until the chocolate has melted completely with the butter. If using a double boiler, make sure the bowl is heat-proof, and place it over a pan with simmering water, and whisk the mixture until the chocolate and butter have melted together.

Add the granulated sugar and whisk to combine.

Add the pistachio paste, and if desired, add green food coloring to make the batter a bright color.

Add the egg, egg yolks and whisk until incorporated.

Add the flour and salt to the bowl, mix on low to combine. You can finish mixing with a spatula to avoid over mixing the batter.

Remove the blondies from the oven.

Mix the heavy cream and pistachio paste together.

Heat up the mixture until 175ºF, or until very hot in the microwave, before it comes to a boil.

Pour the cream into the white chocolate through a strainer, to retain any bits of pistachio nuts in order to obtain a smooth ganache. Squeeze the mixture well to ensure only the bits of nuts are being retained by the strainer. Mix with a whisk until the chocolate melts entirely.

Let the ganache cool down at room temperature for about 1 hour. You can place it in the fridge and stir every 5 minutes or so to speed up the setting process.

Spread the ganache over the cooled blondies.

Let it set for about 10 minutes in the fridge or 40 minutes at room temperature. Slice and serve.

Store the blondies in an air-tight container, in the fridge, for up to 3 days. Let them come to room temperature by letting them sit on the counter for 20 minutes before serving.