Combine all the banana curd ingredients EXCEPT butter in the bowl of a food processor and process until fully combined. Transfer mixture to a medium saucepan set over medium heat and add the butter.

Cook, stirring constantly, until mixture is thick enough to coat the back of a spoon—do not boil. This will take about 8 to 10 minutes.

Pour through a fine mesh sieve into a mixing bowl and press plastic wrap directly onto the top of the curd to prevent a skin from forming. (You may need to use a spatula to press the curd through the sieve.)

Cool curd to room temperature, then refrigerate until chilled (about 2 hours). Can be made up to 3 days ahead.

Preheat the oven to 350 degrees. Line 20 to 22 muffin cups with paper liners and set aside.

Whisk the coffee (or wateand cocoa powder in a small mixing bowl. Cool to room temperature.

Sift the flour, baking soda, baking powder, and salt together into a second mixing bowl and set aside.

Beat the butter, sugar, eggs, and vanilla together in a large mixing bowl with an electric mixer set to high speed. Continue beating on high until mixture is light and fluffy, scraping down the sides and bottom of the bowl as needed—this will take about 5 minutes.

Turn mixer to low and add the flour mixture in 3 additions, alternating with the cocoa mixture, beginning and ending with the flour.

Divide batter among the prepared muffin cups, filling each 2/3 full. Bake 15 to 20 minutes, until a cake tester inserted into the center of the cupcakes comes out clean.

Cool cupcakes completely in pan for 20 minutes then remove to a wire rack to cool completely before filling and frosting.

Beat the butter with an electric mixer on medium speed until very smooth. Add 4 cups of powdered sugar, the cocoa powder, 6 tablespoons milk, and salt and beat to combine.

Scrape down the sides and bottom of the bowl, then turn mixer to medium and beat for a minute or two, until frosting is smooth. Add additional powdered sugar if frosting is too thin and additional milk if frosting is too thick.

Use a small paring knife to cut a hole about 1 inch across and 1 inch deep in each cupcake. Reserve the cake you cut out of the holes.

Pipe or spoon about 2 teaspoons of banana curd into each cupcake. Trim the cake you removed from the holes so it will fit over the filling as a sort of plug, then replace the plug in the hole.

Spread or pipe the chocolate frosting over each cupcake, then top slice of banana if desired. Serve.