For the cupcakes, preheat oven to 350F and line a muffin tray with 6 paper liners.
Use a handheld electric mixer to beat together the butter and sugar in a medium bowl. Once combined, beat in the egg and mix until light and fluffy. Beat in the Guinness, vanilla extract, and vanilla bean paste.
Whisk the flour, baking powder, and salt together in a small bowl. Add the dry ingredients to the wet and mix only until combined, scraping down the sides of the bowl as necessary, and being careful not to over-mix.
Pour the batter into the prepared tray and bake until a toothpick inserted inside comes out clean, about 18 to 20 minutes. Cool completely before frosting.
Once the cupcakes are cool, make the buttercream. Use a handheld electric beater to mix the butter, dissolved coffee, and caramel extract together in a medium bowl. Add the powdered sugar and continue beating while gradually adding milk until the buttercream is smooth and creamy. (You may need a little more milk or powdered sugar to achieve the right consistency.)
Frost the cupcakes with the buttercream and decorate with chocolate sprinkles if desired.