Preheat the oven to 350℉. Line a baking sheet with parchment and set aside. Adjust the oven rack to the middle of the oven as you'll be baking these cookies one tray at a time.

In a large bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In a large bowl with an electric mixer, cream together the butter and peanut butter until smooth and creamy. Mix in the sugar until well combined. Beat in the egg and vanilla and beat until just combined.

Gradually beat the dry ingredients into the peanut butter batter until just combined.

Use a spoon or small scoop and drop 1" balls of dough onto the baking sheet. Bake for 10-12 minutes. You don't want the cookies fully baked, but need to be soft in the middle with the edges a light golden brown. Cool the sheet on a baking rack for two minutes. Then transfer the cookies to a baking rack and bake the next round of cookies. Repeat until all the batter is used. You should end up with 24 cookies--or at least an equal amount of cookies.

In a large bowl and using and electric mixer, beat the peanut butter and butter together. Beat in the confectioner's sugar until the batter is dry and crumbly. Add the cream on Tablespoon at a time. This will be a thick batter but you can add more cream if you need to.

Spread a thin layer of the peanut butter filling on the back sides of 12 cookies. Spread a layer of blackberry jam on the back sides of the remaining 12 cookies. Then make sandwiches with the filling cookies and the jam cookies, 1 per sandwich. Squish the cookies so the jam appears around the cookie edges.