Preheat oven to 350°F (175°C). Line a 12-cup muffin tin with liners.

For the rhubarb filling: In a medium saucepan, combine chopped rhubarb, 0.25 cup honey, and cornstarch. Cook over medium heat, stirring constantly, until rhubarb has softened and the mixture has thickened, about 5-7 minutes. Stir in 0.5 teaspoon vanilla extract. Remove from heat and let cool completely.

For the cupcakes: In a medium bowl, whisk together flour, baking powder, and salt. In a large bowl, cream softened butter, 0.5 cup granulated sugar, and 0.25 cup honey until light and fluffy.

Beat in eggs one at a time, then stir in 1 teaspoon vanilla extract. Gradually add the dry ingredients to the wet ingredients, alternating with the milk, beginning and ending with the dry ingredients. Mix until just combined.

Spoon about 1 tablespoon of the cupcake batter into each muffin liner. Top with about 1 tablespoon of the cooled rhubarb filling, then cover with another tablespoon of batter. Don't overfill.

Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean. Let cool in the muffin tin for 5 minutes before transferring to a wire rack to cool completely.

For the frosting: In a large bowl, beat softened butter until creamy. Gradually add sifted powdered sugar, alternating with 0.25 cup honey, until smooth and well combined. Stir in 1 tablespoon milk and 0.5 teaspoon vanilla extract until the frosting reaches your desired consistency.

Once cupcakes are completely cool, frost them generously with the honey frosting.