Preheat oven to 350F (180C). Butter an 8-inch wide, 3" tall spring-form pan, line the bottom with parchment paper, dust the sides with flour.

In a large heatproof bowl, melt the chocolate, butter, and salt in a microwave

Add the eggs, one at a time, and mix until fully incorporated.

Fold the flour and halved peanut butter cups into the batter, until just combined.

Pour the batter into the prepared pan. Bake for 15 minutes.  (At this time begin to prepare the cheesecake layer, see below)

Remove the brownie bottom from the oven, turn the oven down to 300 degrees. Let the brownie layer cook for at least 15 minutes.

In the bowl of a stand mixer, or in a large bowl using an electric hand mixer, beat the cheesecake and sugar until creamy and smooth. Scrape down the sides and bottom well to make sure you don't leave any lumps behind.

Add the peanut butter, cornstarch, salt, and vanilla extract. Mix on medium sped until well combined. Scraping down the sides and bottom again will help you avoid lumps.

Fold or manually whisk in the eggs, one at the time. It is very important not to over mix at this point. The eggs should be just incorporated.

Cut 2 pieces of foil that are at least 12" long, lay out in a cross shape. Place the pan in the center of the cross and fold up the foil, creating a tight fit around the pan.

Place the pan in a 9 x 13 pan.

Pour the cheesecake batter over the cooled brownie layer. Pour hot water (hot tap water works), around the cheesecake so that it is halfway up the sides of the spring form pan.

Bake for 60-70 minutes, until the cheesecake is set but still slightly jiggly in the middle when you gently shake it.

Turn off the oven and leave the door ajar, let the cheesecake cool for 15-30 minutes, while still in the oven.

Remove the cheesecake from the oven and water bath, let cool to room temperature completely. This can take up to 4 hours, depending on ambient room temperature.

Carefully remove the cheesecake from the pan by running your knife along the sides. Open the spring and carefully pulling the open form up, away from the cheesecake.

Place the cheesecake on a plate and loosely cover with plastic wrap. Refrigerate at least 8 hours, or over night.

In a large heatproof bowl, melt the chocolate and heavy cream, and salt together in 30-second intervals, stirring between the intervals, until it is completely melted.

Let cool to a spreadable consistency.

In a large heatproof bowl, melt the peanut butter, butter, and salt together in 30-second intervals, stirring between the intervals.

Sift the confectioners' sugar into the bowl, mix well.

Let cool to a spreadable consistency.

Using an offset spatula, lift the cheesecake off the bottom pan. Pick the cheesecake up and carefully remove the parchment paper on the bottom. Place back on the bottom, or directly on a serving plate.

Pour half of the ganache on the center of the cheesecake, using the offset spatula, move the ganache to the edge of the cheesecake and let it side down the sides.  Using the offset spatula, spread the  ganache around the sides. Add the rest of the ganache and completely cover the cake.

Place in the refrigerator for 15 minutes.

Spread the peanut butter glaze on the top and then decorate the top with the peanut butter cups.